WordArt: cap-merge + adaptive segments, atomic atlas swaps, live scale

[apresmoi]

What

Two themes, both improving the WordArt composer (/wordart) and the renderers it sits on:

1 · Fonts geometry perf (lossless, same silhouette)

• Cap convex-merge — composeText/textPolygons now merge the earcut cap triangles into maximal convex polygons at build time. Fewer DOM leaves (e.g. WordArt flat 644→414, "Hi" 132→58) and about half the front/back internal seams gone. Identical outline + holes; many triangle-pairs collapse to the cheapest <b> rect. The old generic merge option is removed (superseded by this always-on pass).
• Adaptive round-edge segments — the round profile picks the fewest segments whose chord error stays under ~0.4% of cap height (the round-over radius is capped, so the default never over-tessellates). Round WordArt 3284→2186 by default, visually identical.

2 · Flicker-free editing (atlas pipeline, react + vue)

• Never-blank double-buffer — the atlas holds the previous bitmap (pre-decoded via Image.decode()) until the new one is ready instead of blanking to shells on every edit, so geometry edits don't flash the textured face.
• Opt-in atomicAtlas on PolyMesh — holds the whole previous frame (geometry + texture together) until the new atlas is decoded, then swaps atomically, cancelling any superseded build. Used by /wordart so a slider release never shows geometry before its texture. Adds an onFrameReady callback for preview hand-off. Mirrored in react + vue.

3 · WordArt workbench UX

• Depth previews live via a CSS scaleZ while dragging and bakes on release (clean swap via atomicAtlas).
• Scale X/Y are now a pure live CSS transform on the mesh (no recompute, no flicker) — they stretch the whole block uniformly and the camera (fitZoom) compensates to stay framed.
• Heavy sliders (segments, simplify, warp, letter spacing, line height) commit on release for a clean atomic swap.
• Dropped the dead "Merge polygons" toggle and a duplicate case clause.

Test plan

• pnpm test — core 949, fonts 45, polycss 584, react 414, vue 415, all green.
• pnpm build — all packages + website clean.
• Manual: dragging every /wordart slider on textured/gradient fills — no blank flash, no destructive swap; Depth + Scale X/Y feel live.

Follow-ups (not in this PR)

• Document the new PolyMesh atomicAtlas / onFrameReady props in the website docs.
• Live letter spacing + line height via per-leaf CSS-var offsets (the dynamic-lighting pattern) — a separate cross-package feature.