.ezy format v3: ZIP container with deduplicated underlay assets

[trailcode]

Summary

• .ezy format v3: Saved projects are a ZIP archive (manifest.json + deduplicated assets/*.rgba underlay blobs). Sketch underlays reference assets by content hash instead of inline base64.
• Ezy_asset_store: Session-lifetime content-addressed registry for underlay RGBA blobs; undo/redo snapshots share assets instead of re-embedding pixels.
• ezy_io: Zip pack/unpack, format sniff, and base64 helpers for Wasm startup storage.
• Backward compatibility: Legacy plain-JSON .ezy files (v1/v2, including rgba_b64 underlays) still load; pixels migrate into the asset store on read.
• GUI refactor: Split gui.cpp (gui_add.cpp, gui_settings.cpp), tidy gui_mode.cpp (shared default-material combo helper), rename gui_modes → mode.
• Extrude UX: Hide permanent sketch nodes and snap annotations during sketch face extrude (consistent with operational axis behavior).

Closes #156

Files changed

• src/ezy_asset_store.h/.cpp, src/ezy_io.h/.cpp — new I/O and asset store
• src/occt_view.*, src/sketch_underlay.*, src/sketch_json.* — v3 save/load and asset refs
• src/gui.cpp, src/gui_add.cpp, src/gui_mode.cpp, src/gui_settings.cpp, src/gui.h — GUI split and cleanup
• src/mode.h/.cpp (renamed from gui_modes)
• tests/sketch_tests.cpp — pack/unpack, dedup, legacy migration tests
• CHANGELOG.md — format v3 entry

Test plan

• Build EzyCad_lib and EzyCad_tests (Debug + Release).
• Run EzyCad_tests — ezy zip round-trip, asset deduplication, legacy rgba_b64 load.
• Manual: save project with image underlay, reopen, verify underlay renders correctly.
• Manual: open older plain-JSON .ezy, save again, confirm v3 ZIP output.
• Manual: sketch face extrude — perm nodes / snap suppressed during tool.
• Verify Options panel material combos still work (Normal, polar dup, sketch extrude).

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