feat(renderer): build-plate surface (solid / procedural generic / consumer texture)

[sobechestnut-dev]

Backlog candidate (2026-07-23 research pass) · triage against E9 #162 (renderer options).

Offer a build-plate surface instead of only the bare wireframe grid. Ship in tiers:

1. Solid shaded plate — a filled, theme-colored plane. Zero assets, kills the "empty grid floor" look, synchronous (safe for the headless renderStill path).
2. Procedural / SVG generic plate + functional overlays — a self-drawn CanvasTexture (no assets, no branding, synchronous, theme-able) plus origin marker, bed dimensions, and exclusion zones drawn from MachineGeometry.excludedRegions (verified present in the contract). Arguably more useful than a photo texture for an analysis tool.
3. Consumer-supplied texture (hook) — a bedSurface option taking a preloaded ImageBitmap/canvas (NOT a URL — CSP + async/still-render). Consumers who want a branded plate own that asset.

IP guardrails (important): do NOT bundle vendor plate textures — brand names/logos are trademarks and vendor bitmap assets are copyrighted (also bloat). Generic self-drawn surfaces and functional overlays (exclusion zones, origin, dimensions) are fine — they're our original work / parsed data. Prime/skirt/brim are not plate decorations; they're already toolpath geometry (FeatureRole.Skirt/Brim).

Composition: drapes over the discovered bed shape (rect/circular/polygon); slots into the DD-009 Theme as an optional bedSurface field.

Implementation gated by DD/E9 triage per project governance.