Sketch-bounds OCCT shader grid with padding and tools/ dev helpers

[trailcode]

Summary

• Sketch-bounds shader grid: Size the 3D reference grid from the active sketch node bounds plus a configurable Grid padding margin instead of fixed extent X/Y. Uses OCCT Aspect_GridParams on the sketch plane and refreshes when sketch geometry changes.
• Settings migration: Replace grid_graphic_x_size / grid_graphic_y_size with grid_padding; legacy JSON loads old half-extent as padding.
• Depth / presentation: Grid Z offset tuned so coplanar sketch geometry draws in front of grid lines.
• Dev tooling: Add tools/pimpl_gen.py (PIMPL forwarding stub generator), move pbf-to-png scripts to tools/, and show tools/ in the CMake IDE tree. Extract ezy_geom types to src/utl_geom_boost.inl.

Closes #154

Files changed

• src/occt_view.cpp, src/occt_view.h — shader grid layout, padding-based sizing, refresh hook
• src/gui_settings.cpp — Settings UI and JSON read/write for grid_padding
• src/sketch.cpp — refresh grid after face updates on current sketch
• docs/usage-settings.md, res/ezycad_settings.json — user docs and defaults
• tools/pimpl_gen.py, tools/pbf-to-png.* — dev helpers (moved from scripts/)
• CMakeLists.txt — IDE visibility for tools/
• src/utl_geom.h, src/utl_geom_boost.inl — geometry types extracted to .inl

Test plan

• Build Release: EzyCad, EzyCad_lib
• Manual: sketch geometry grows — grid expands with bounds + padding
• Manual: adjust Grid padding, step, colors, visibility in Settings
• Manual: grid lines sit behind coplanar sketch edges on the plane
• Manual: load settings JSON with legacy grid_graphic_x_size only
• Optional: python tools/pimpl_gen.py --list src/<header>.h

Notes

• Branch commit messages are WIP placeholders; squash or rewrite on merge if desired.
• tools/__pycache__/pimpl_gen.cpython-312.pyc should probably be dropped from the branch and added to .gitignore before merge.
• Stale agent drafts (agents/issues/017-..., agents/prs/012-...) on this branch describe merged sketch-undo work (Sketch undo: stable sketch ids for delta resolve, operation-axis undo, and undo stack byte budget #150/Sketch undo: stable sketch ids for delta resolve, operation-axis undo, and undo stack byte budget #151) and should be removed or replaced in a follow-up.