Add 3D/embed/perf features

[CSSFrancis]

Major additions across rendering, embedding, and performance:

Rendering & plot types

• True-colour RGB imshow ((H,W,3|4) arrays)
• 3D voxels geometry + draggable PlaneWidget slice selectors
• scatter3d per-point colours, set_highlight, reference sphere, bounds
• Turntable 3D camera (matplotlib azim/elev; reaches any viewpoint)
• Mini-TeX label formatting (superscripts/Greek/symbols) + fontsize control
• IPF and voxel-grain explorer examples; label-formatting example

Embedding (no Jupyter required)

• anyplotlib.embed: figure_state/to_html/save_html/esm_path + FigureBridge
• figure_esm.js mount() API for Electron / plain web pages
• docs/embedding.rst guide

WebGPU (progressive enhancement, canvas fallback)

• Instanced points + voxels on the GPU via gpu="auto"; airtight Canvas2D fallback (no navigator.gpu / null adapter / device loss). No JS deps.
• Hardware-verified on NVIDIA Pascal; WEBGPU_PLAN.md tracks phases.

Performance (smoothness under Pyodide)

• Figure.batch() coalesces panel pushes (one per panel per frame)
• Geometry channel: heavy buffers ride a separate hash-gated trait, re-sent only on change — view updates (highlight/camera/planes) no longer retransmit geometry. Voxel explorer per-frame traffic -65%.
• Voxel sprite cache + typed-array projection; RGB imshow skips LUT rebuild

Packaging

• LICENSE (MIT), wheel excludes tests/baselines, py3.13 classifier
• RELEASE_PLAN.md; committed uv.lock

Tests: +~90 across labels, embed, RGB, voxels/planes, GPU fallback, batch, geometry channel. Full suite 1323 passed / 58 skipped.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.15385% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.63%. Comparing base (8a9d6fd) to head (d9c8ef5).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
anyplotlib/embed.py	93.61%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
- Coverage   90.84%   90.63%   -0.21%     
==========================================
  Files          31       33       +2     
  Lines        2501     2744     +243     
==========================================
+ Hits         2272     2487     +215     
- Misses        229      257      +28     

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