Add parallelism to sequential fitting

.github/copilot-instructions.md

@@ -42,7 +42,8 @@
   and UPPER_SNAKE_CASE for constants.
 - Use `from __future__ import annotations` in every module.
 - Type-annotate all public function signatures.
-- Docstrings on all public classes and methods (numpy style).
+- Docstrings on all public classes and methods (numpy style). These must
+  include sections Parameters, Returns and Raises, where applicable.
 - Prefer flat over nested, explicit over clever.
 - Write straightforward code; do not add defensive checks for unlikely
   edge cases.
@@ -147,6 +148,8 @@
   `docs/architecture/architecture.md`.
 - After changes, run linting and formatting fixes with `pixi run fix`.
   Do not check what was auto-fixed, just accept the fixes and move on.
+  Then, run linting and formatting checks with `pixi run check` and
+  address any remaining issues until the code is clean.
 - After changes, run unit tests with `pixi run unit-tests`.
 - After changes, run integration tests with
   `pixi run integration-tests`.

docs/architecture/architecture.md

@@ -188,7 +188,7 @@ GuardedBase
 └── GenericDescriptorBase               # name, value (validated via AttributeSpec), description
     ├── GenericStringDescriptor         # _value_type = DataTypes.STRING
     └── GenericNumericDescriptor        # _value_type = DataTypes.NUMERIC, + units
-        └── GenericParameter            # + free, uncertainty, fit_min, fit_max, constrained, uid
+        └── GenericParameter            # + free, uncertainty, fit_min, fit_max, constrained
 ```
 
 CIF-bound concrete classes add a `CifHandler` for serialisation:
@@ -714,12 +714,13 @@ Projects are saved as a directory of CIF files:
 ```shell
 project_dir/
 ├── project.cif          # ProjectInfo
-├── analysis.cif         # Analysis settings
 ├── summary.cif          # Summary report
 ├── structures/
 │   └── lbco.cif         # One file per structure
-└── experiments/
-    └── hrpt.cif         # One file per experiment
+├── experiments/
+│   └── hrpt.cif         # One file per experiment
+└── analysis/
+    └── analysis.cif     # Analysis settings
 ```
 
 ### 7.3 Verbosity
@@ -919,6 +920,10 @@ project.experiments['xray_pdf'].peak_profile_type = 'gaussian-damped-sinc'
 - `DatablockItem` = one CIF `data_` block, `DatablockCollection` = set
   of blocks.
 - `CategoryItem` = one CIF category, `CategoryCollection` = CIF loop.
+- **Free-flag encoding**: A parameter's free/fixed status is encoded in
+  CIF via uncertainty brackets. `3.89` = fixed, `3.89(2)` = free with
+  esd, `3.89()` = free without esd. There is no separate list of free
+  parameters; the brackets are the single source of truth.
 
 ### 9.2 Immutability of Experiment Type
 

docs/architecture/issues_closed.md

@@ -4,6 +4,56 @@ Issues that have been fully resolved. Kept for historical reference.
 
 ---
 
+## Implement `Project.load()`
+
+**Resolution:** implemented `Project.load(dir_path)` as a classmethod
+that reads `project.cif`, `structures/*.cif`, `experiments/*.cif`, and
+`analysis/analysis.cif` (with fallback to `analysis.cif` at root for
+backward compatibility). Reconstructs the full project state including
+alias parameter references via `_resolve_alias_references()`.
+Integration tests verify save → load → parameter comparison and save →
+load → fit → χ² comparison. Also used by `fit_sequential` workers to
+reconstruct projects from CIF strings.
+
+---
+
+## Eliminate Dummy `Experiments` Wrapper in Single-Fit Mode
+
+**Resolution:** refactored `Fitter.fit()` and `_residual_function()` to
+accept `experiments: list[ExperimentBase]` instead of requiring an
+`Experiments` collection. `Analysis.fit()` passes
+`experiments_list = [experiment]` in single-fit mode and
+`list(experiments.values())` in joint-fit mode. Removed the
+`object.__setattr__` hack that forced `_parent` on the dummy wrapper.
+
+---
+
+## Replace UID Map with Direct References and Auto-Apply Constraints
+
+**Resolution:** eliminated `UidMapHandler` and random UID generation
+from parameters entirely. Aliases now store a direct object reference to
+the parameter (`Alias._param_ref`) instead of a random UID string.
+`ConstraintsHandler.apply()` uses the direct reference — no map lookup.
+For CIF serialisation, `Alias._param_unique_name` stores the parameter's
+deterministic `unique_name`. `_minimizer_uid` now returns
+`unique_name.replace('.', '__')` instead of a random string.
+
+Also added `enable()`/`disable()` on `Constraints` with auto-enable on
+`create()`, replacing the manual `apply_constraints()` call.
+`Analysis._update_categories()` now always syncs handler state from the
+current aliases and constraints when `constraints.enabled` is `True`,
+eliminating stale-state bugs (former issue #4). `_set_value_constrained`
+bypasses validation like `_set_value_from_minimizer` since constraints
+run inside the minimiser loop. `Analysis.fit()` calls
+`_update_categories()` before collecting free parameters so that
+constrained parameters are correctly excluded.
+
+API change: `aliases.create(label=..., param_uid=...uid)` →
+`aliases.create(label=..., param=...)`. `apply_constraints()` removed;
+`constraints.create()` auto-enables.
+
+---
+
 ## Dirty-Flag Guard Was Disabled
 
 **Resolution:** added `_set_value_from_minimizer()` on

docs/architecture/issues_open.md

@@ -10,25 +10,6 @@ needed.
 
 ---
 
-## 1. 🔴 Implement `Project.load()`
-
-**Type:** Completeness
-
-`save()` serialises all components to CIF files but `load()` is a stub
-that raises `NotImplementedError`. Users cannot round-trip a project.
-
-**Why first:** this is the highest-severity gap. Without it the save
-functionality is only half useful — CIF files are written but cannot be
-read back. Tutorials that demonstrate save/load are blocked.
-
-**Fix:** implement `load()` that reads CIF files from the project
-directory and reconstructs structures, experiments, and analysis
-settings.
-
-**Depends on:** nothing (standalone).
-
----
-
 ## 2. 🟡 Restore Minimiser Variant Support
 
 **Type:** Feature loss + Design limitation
@@ -83,31 +64,6 @@ exactly match `project.experiments.names`.
 
 ---
 
-## 4. 🔴 Refresh Constraint State Before Automatic Updates and Fitting
-
-**Type:** Correctness
-
-`ConstraintsHandler` is only synchronised from `analysis.aliases` and
-`analysis.constraints` when the user explicitly calls
-`project.analysis.apply_constraints()`. The normal fit / serialisation
-path calls `constraints_handler.apply()` directly, so newly added or
-edited aliases and constraints can be ignored until that manual sync
-step happens.
-
-**Why high:** this produces silently incorrect results. A user can
-define constraints, run a fit, and believe they were applied when the
-active singleton still contains stale state from a previous run or no
-state at all.
-
-**Fix:** before any automatic constraint application, always refresh the
-singleton from the current `Aliases` and `Constraints` collections. The
-sync should happen inside `Analysis._update_categories()` or inside the
-constraints category itself, not only in a user-facing helper method.
-
-**Depends on:** nothing.
-
----
-
 ## 5. 🟡 Make `Analysis` a `DatablockItem`
 
 **Type:** Consistency
@@ -150,24 +106,6 @@ effectively fixed after experiment creation.
 
 ---
 
-## 7. 🟡 Eliminate Dummy `Experiments` Wrapper in Single-Fit Mode
-
-**Type:** Fragility
-
-Single-fit mode creates a throw-away `Experiments` collection per
-experiment, manually forces `_parent` via `object.__setattr__`, and
-passes it to `Fitter`. This bypasses `GuardedBase` parent tracking and
-is fragile.
-
-**Fix:** make `Fitter.fit()` accept a list of experiment objects (or a
-single experiment) instead of requiring an `Experiments` collection. Or
-add a `fit_single(experiment)` method.
-
-**Depends on:** nothing, but simpler after issue 5 (Analysis refactor)
-clarifies the fitting orchestration.
-
----
-
 ## 8. 🟡 Add Explicit `create()` Signatures on Collections
 
 **Type:** API safety
@@ -339,21 +277,18 @@ re-derivable default.
 
 ## Summary
 
-| #   | Issue                                      | Severity | Type                    |
-| --- | ------------------------------------------ | -------- | ----------------------- |
-| 1   | Implement `Project.load()`                 | 🔴 High  | Completeness            |
-| 2   | Restore minimiser variants                 | 🟡 Med   | Feature loss            |
-| 3   | Rebuild joint-fit weights                  | 🟡 Med   | Fragility               |
-| 4   | Refresh constraint state before auto-apply | 🔴 High  | Correctness             |
-| 5   | `Analysis` as `DatablockItem`              | 🟡 Med   | Consistency             |
-| 6   | Restrict `data_type` switching             | 🔴 High  | Correctness/Data safety |
-| 7   | Eliminate dummy `Experiments`              | 🟡 Med   | Fragility               |
-| 8   | Explicit `create()` signatures             | 🟡 Med   | API safety              |
-| 9   | Future enum extensions                     | 🟢 Low   | Design                  |
-| 10  | Unify update orchestration                 | 🟢 Low   | Maintainability         |
-| 11  | Document `_update` contract                | 🟢 Low   | Maintainability         |
-| 12  | CIF round-trip integration test            | 🟢 Low   | Quality                 |
-| 13  | Suppress redundant dirty-flag sets         | 🟢 Low   | Performance             |
-| 14  | Finer-grained change tracking              | 🟢 Low   | Performance             |
-| 15  | Validate joint-fit weights                 | 🟡 Med   | Correctness             |
-| 16  | Persist per-experiment `calculator_type`   | 🟡 Med   | Completeness            |
+| #   | Issue                                    | Severity | Type                    |
+| --- | ---------------------------------------- | -------- | ----------------------- |
+| 2   | Restore minimiser variants               | 🟡 Med   | Feature loss            |
+| 3   | Rebuild joint-fit weights                | 🟡 Med   | Fragility               |
+| 5   | `Analysis` as `DatablockItem`            | 🟡 Med   | Consistency             |
+| 6   | Restrict `data_type` switching           | 🔴 High  | Correctness/Data safety |
+| 8   | Explicit `create()` signatures           | 🟡 Med   | API safety              |
+| 9   | Future enum extensions                   | 🟢 Low   | Design                  |
+| 10  | Unify update orchestration               | 🟢 Low   | Maintainability         |
+| 11  | Document `_update` contract              | 🟢 Low   | Maintainability         |
+| 12  | CIF round-trip integration test          | 🟢 Low   | Quality                 |
+| 13  | Suppress redundant dirty-flag sets       | 🟢 Low   | Performance             |
+| 14  | Finer-grained change tracking            | 🟢 Low   | Performance             |
+| 15  | Validate joint-fit weights               | 🟡 Med   | Correctness             |
+| 16  | Persist per-experiment `calculator_type` | 🟡 Med   | Completeness            |