feat(core): Illustration primitive + svg-illustration skill

[daniellee-ux]

Summary

Designs could previously only fake illustrative art with CSS shapes (gradient Ellipses, emoji Icons, bordered Boxes). This PR adds real inline-SVG illustration support:

• New <Illustration> primitive — a full-bleed host for hand-authored SVG with its own data-ox-type, so it avoids the Inspector's icon-typed behaviors (the blank text-popover whose empty save would delete SVG children, and the color swatch writing an inert prop). Its background prop paints a mat behind the artwork; board.css sizes a direct <svg> child to fill the box.
• Inspector — background handled everywhere fill is: fillProp routing, currentColorOf, fillIndeterminate, and one shared display name across the swatch label, aria-label, toast, and both commitProps edit labels.
• LayersPanel — glyph for the new type.
• New svg-illustration skill (canonical + committed demo mirror) — hosting contract, JSX-not-raw-SVG casing, useId()-prefixed defs ids, viewBox bleed margins for filters, theme-token + color-mix() palette derivation, primitives-first build order, render-look-fix loop, and a compact technique checklist. Referenced from canva-authoring; Icon is rescoped to small glyphs/marks.
• Docs — skill lists and primitive enumerations updated across AGENTS.md/CLAUDE.md, READMEs, and the init template; apply-comments prop list gains background.

Deliberately deferred

overflow.ts occlusion checks (either direction) for illustration objects — the skill's render-look-fix step covers this manually; the lint's crowding check already exempts the new type by never listing it. Tracked as a fast-follow.

Verification

• npm run typecheck, npx tsc --noEmit -p apps/demo, npm run build — all pass
• npm run sync + check:sync — committed mirror matches canonical; .claude/skills/ stays untracked
• opencanva init into a scratch dir picks up the new skill automatically
• In-browser: drag/resize/rotate/z write-back targets the <Illustration> JSX; no text popover for the new type; swatch writes background and labels/toast agree; useId() ids don't collide across two instances + live/thumbnail double render; lint clean incl. no crowding false-positive for full-bleed placement
• Export fidelity: PNG (window.__ox.pngDataUrl()), standalone SVG, and real PDF export all match the live preview incl. gradients, color-mix() in presentation attributes, clipPath, and expanded-region filters
• End-to-end comment flow proven: @canva-comment on an <Illustration> → apply-comments → SVG edit → render-look-fix → lint clean

🤖 Generated with Claude Code

[daniellee-ux]

Codex review (codex exec review --base main, codex-cli 0.139.0) verdict:

The changes add the Illustration primitive, inspector support for its background mat, layers labeling, and synchronized skill/docs updates without introducing an actionable correctness issue. Typecheck passed in the available sandbox.

No actionable findings — merging as-is.