perf: reuse projection icons during stable refresh

[10-X-eng]

Outcome

Eliminates measured duplicate icon work during the Tree and Feature Timeline refresh at the end of large publications.

• Cache named SVG rasterization by resolved path, rendered pixel size, and exact color map.
• Reuse the already-correct legacy Tree icon when the semantic browser renders the same document object with the same effective visibility.
• Reuse exact Timeline icon variants within one rebuild using the Qt icon identity and complete visual-state bits.
• Keep cache lifetime and ownership narrow; no persistent projection state was added.

Why

The representative 307-member Assembly trace proved that the final 1.864-second UI stall was dominated by duplicate Tree and Timeline icon generation, not semantic classification or tessellation.

Red / green evidence

Red baseline (build/run_vibescript_publication_307_native_trace.ps1):

• Correct 307-member publication, but maximum GUI heartbeat gap: 1,863.733 ms (100 ms contract failed).
• Browser item creation: 463.308 ms / 321.
• Tree icon generation: 912.865 ms / 642.
• Timeline icon transformation: 393.617 ms / 312.

Green, same fixture and command against the exact rebuilt Rattler/MSVC DLL:

• Correct publication: 307 members / 322 objects.
• Maximum GUI heartbeat gap: 644.622 ms (65.4% lower; remaining batching work is explicitly not claimed complete).
• Browser item creation: 4.273 ms / 321 (99.1% lower).
• Tree icon generation: 287.627 ms / 321 (68.5% lower).
• Timeline icon transformation: 2.351 ms / 6 (99.4% lower).
• Longest cooperative publication unit: 93.870 ms.

Focused test:

.pixi\envs\default\python.exe -m pytest src\Mod\VibeCAD\vibecad_tests\test_performance.py -q

Result: 12 passed.

Exact native build:

cmake --build <rattler-work-build> --target FreeCADGui --parallel 8

Result: passed with MSVC, followed by the native benchmark above. A red compile caught an incorrect QIcon/QPixmap argument during simplification; the corrected target rebuilt and linked cleanly before measurement.

Risk and non-goals

Risk is limited to icon caching/reuse. Failed SVG loads are not cached. Keys include every input that changes rendered output. This PR deliberately does not claim to solve the remaining 645 ms final projection stall; bounded GUI-thread widget batching is the next measured step.

Compatibility checklist

• Existing public functions/APIs still present and behaviorally compatible.
• No preference keys, tool names, or schema fields renamed or removed.
• Defaults and visible behavior are unchanged.
• No deprecations.
• No breaking changes.

[10-X-eng]

Closed at the repository owner request; this performance implementation is being discarded.