Fix instruction browser review findings

[hbmartin]

Summary

Follow-up to #20 addressing all four code-review findings:

• Error loads exit Instructions mode — _show_error now resets the view mode, syncs the mode select, and re-syncs tab visibility, so a failed load restores the whole-model controls instead of stranding empty section/step dropdowns.
• Step labels cached and guarded — per-section occurrence counts are computed once per section selection instead of re-expanding every prior step on each render (one expansion per step change instead of ~seven). The labels are validated against the cumulative occurrence count before use; if pyldraw3's concatenation invariant ever drifts, the Pieces table falls back to blank step labels instead of dying on the strict=True zip.
• Directive cells render losslessly — the Data and Raw LDraw columns are wrapped in Text(...) so bracketed content (JSON arrays, [tag]-style comment text) displays literally instead of going through Rich markup parsing.
• _instruction_selection fallback syncs state — the defensive fallback to step 1 now updates _selected_instruction_step to match what it returns.

New tests cover the two gaps flagged in review: [/] clamping at section bounds, and loading a broken file while in Instructions mode.

Validation

• uv run pytest (77 passed)
• uv run ruff check . / uv run ruff format --check src tests
• uv run ty check src tests
• uv run pyrefly check src tests
• No snapshot changes — rendered output for all snapshotted views is identical

🤖 Generated with Claude Code

---

Summary by cubic

Fixes the instruction browser: error loads now fully reset to whole-model mode, directive cells render literally, and step labels are stabilized with cached counts to avoid crashes and extra work. Also reuses precomputed occurrences in stats to cut recomputation.

• Bug Fixes

  • On load errors, fully exit Instructions mode: reset to whole-model key, clear submodel selector, sync mode select, and hide instruction tabs.
  • Render directive Data/Raw cells with Text so bracketed content displays literally.
  • Stabilize step labels; guard against cumulative occurrence drift and sync _selected_instruction_step in the fallback.

• Refactors

  • Cache per-section step occurrence counts; reuse across renders and clear on rebuild/error.
  • Pass counts into InstructionDetails.show_step and reuse precomputed model occurrences in the stats panel to avoid extra expansions.
  • Use typed query_one(expect_type=...) lookups.

^{Written for commit a5d3e3f. Summary will update on new commits.}

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[codecov]

Codecov Report

❌ Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.26%. Comparing base (5f31a3e) to head (321a02b).

Files with missing lines	Patch %	Lines
src/pyldraw3_tui/screens/model.py	86.36%	3 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           pyldraw-updates      #21      +/-   ##
===================================================
+ Coverage            87.16%   87.26%   +0.10%     
===================================================
  Files                   30       30              
  Lines                 1441     1461      +20     
===================================================
+ Hits                  1256     1275      +19     
- Misses                 185      186       +1     

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