perf: render shapes via PathCollection of pre-built Paths (avoid per-shape Patch construction) #733

Reframed after profiling (the original to_rgba framing was wrong — it's end-to-end noise).

Root cause

render_shapes (matplotlib) is dominated by per-shape Patch construction, identical across no-color / categorical / continuous color (cProfile, 200k circles, ~23 s):

• _build_shape_patches (_geometry.py): ~13 s — one mpatches.Circle/Polygon per shape; each Patch.__init__ resolves default colors via to_rgba (~4×/patch) + recomputes a per-patch transform.
• PatchCollection.set_paths: ~6 s — converting all N patches to Paths.

Fix: PathCollection of pre-built Paths

Build Path objects (not Patch objects) and return a matplotlib.collections.PathCollection. These are exactly the Paths PatchCollection.set_paths produces internally, so it's byte-identical (same backend), preserves holes (compound paths) and the existing per-path affine step, keeps all color/alpha/outline logic, is one code path, and needs no signature change.

Measured (200k circles, build+draw): ~2.6× faster, ~5.8× less memory (11.7 s/149 MB vs 30.1 s/867 MB), color-mode-independent.

Rejected: EllipseCollection/PolyCollection — incompatible with the coordinate-system affine applied per-path (EllipseCollection has one shared read-only path), PolyCollection can't represent holes, and EllipseCollection tessellates circles coarsely. Not byte-identical.

Plan: plans/issue-733.md. Datashader + as_points paths already optimized — untouched.