feat(plot2d): multi-image layers with per-layer cmap, clim, and alpha

[CSSFrancis]

Summary

Stacked on #28 (retarget to main after it merges).

• plot2d.add_layer(data, cmap=, alpha=, clim=, visible=) → Layer handle with set / set_data / remove; plot2d.layers is z-ordered.
• Layers draw bottom-up above the base image with the base's exact fit-rect + zoom/pan transform, under axes/markers/widgets, and composite in Canvas2D even over a WebGPU base. Per-layer LUT bitmaps are cached by (pixels, cmap, clim); exportPNG captures layers for free.
• Transport: layer pixels ride dynamic geom keys (layer_<id>_b64) — _GEOM_KEYS is now a property unioning the fixed set with live layer keys; the Electron binary route ships each layer as its own PLOTBIN frame; resolve_pixel_tokens materialises layer base64 so save_html snapshots stay self-contained.
• Layers and tile mode exclude each other (enforced both directions); layer shape must match the base (H, W).

Motivation: SpyDE image overlays (live MDI layering + composed report figures), where each layer needs its own colormap/clim/alpha and frames swap per navigator move via layer.set_data.

Tests

30 unit + 5 Playwright (alpha-blend pixel assertions, visibility toggle, set_data swap, WebGPU-base + Canvas2D-layer readback, exportPNG-includes-layers). Regression across test_plot2d/ + test_layouts/ + test_embed/: 502 passed, 6 pre-existing baseline skips.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.87179% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.74%. Comparing base (0b74e7c) to head (c766c47).

Files with missing lines	Patch %	Lines
anyplotlib/plot2d/_plot2d.py	95.80%	6 Missing ⚠️
anyplotlib/plot2d/_layer.py	91.30%	4 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           feat/export-png      #29      +/-   ##
===================================================
+ Coverage            88.97%   89.74%   +0.76%     
===================================================
  Files                   38       39       +1     
  Lines                 3511     3696     +185     
===================================================
+ Hits                  3124     3317     +193     
+ Misses                 387      379       -8     

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