test(batch10-stage0): window-mixin composition guardrails

[yilibinbin]

Batch 10 — Stage 0: window-mixin composition guardrails (tests only)

The first, lowest-risk stage of the ExtrapolationWindow decomposition plan
(docs/BATCH10_WINDOW_DECOMPOSITION_PLAN.md). No production code changes —
pure characterization tests that make any future mixin split provably
behavior-preserving. Every check encodes a failure mode surfaced by the
two-model plan review (Codex/gpt-5.5 + Antigravity/Gemini) and re-verified
against the code.

tests/test_window_mixin_composition_guardrails.py:

1. MRO frozen — a base re-order or a shim forgetting its sub-mixins silently
   changes method precedence.
2. Qt bases precede all mixins — documents the severance ordering.
3. No mixin overrides a Qt event handler — QMainWindow's C++ wrapper does
   not cooperatively super(), so a closeEvent/resizeEvent/… override in a
   mixin is silently unreachable (Gemini).
4. No unexpected duplicate method names across sibling mixins — splitting a
   monolith reverses left-to-right shadowing precedence; only the known
   intentional shim override (_on_fit_finished) is allowlisted (Codex+Gemini).
5. No __init__ in mixins — construction stays driven by the window (Codex).
6. Test-imported internals stay importable — a split must re-export moved
   internals like _statistics_raw_table_preserving_cells (Codex).

Test plan

• Each behavioral guard is RED-proven (injecting a Qt-event override /
  duplicate name / mixin __init__ makes its test fail).
• ruff clean; 70 neighborhood tests pass; no production code touched.

Stages 1/3 (the actual splits) are separate future PRs, gated by these
guardrails. Stage 2 (cross-family run controller) and Stage 4 (Protocol) are
NO-GO per the review.

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Run ID: 04899227-9d76-48aa-abdd-d1c7088ab106

📥 Commits

Reviewing files that changed from the base of the PR and between cdc9604 and 1ddf9ae.

📒 Files selected for processing (1)

• tests/test_window_mixin_composition_guardrails.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/batch10-stage0

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