Add docs/internals/ pipeline developer reference (issue #643)

- Move docs/calibration_internals.ipynb → docs/internals/calibration_package_internals.ipynb
- Add docs/internals/data_build_internals.ipynb: Stage 1 coverage — clone creation with real assign_random_geography() on 20 records, source imputation concept demo, PUF cloning toy walkthrough
- Add docs/internals/local_dataset_assembly_internals.ipynb: Stages 3–4 — Hard Concrete L0 math, λ preset comparison, weight expansion reference, diagnostics column guide
- Add docs/internals/README.md: navigation index + §9 pipeline orchestration (run ID format, Modal volumes, step dependency graph, resume logic, HuggingFace artifact paths, meta.json structure)
- Extend calibration_package_internals with Part 4 (matrix assembly per-state, domain constraints) and Part 5 (takeup randomization cross-stage demo)
- All notebooks execute with zero errors under --allow-errors; toy inputs complete in <30s
- Add changelog fragment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: juaristi22

changelog.d/add-calibration-pipeline-internals-docs.added.md

@@ -0,0 +1 @@
+Add `docs/internals/` developer reference: three notebooks covering all nine pipeline stages (Stage 1 data build, Stage 2 calibration matrix assembly, Stages 3–4 L0 optimization and H5 assembly) plus a README with pipeline orchestration reference, run ID format, Modal volume layout, and HuggingFace artifact paths.