v3.4.2: color slot type in composer slot inspector

[heznpc]

Spec: docs/v3-cas-spec.md (v3.4.x line). Builds on #24/#28/#30.

Surface

• Add `color` slot type to the composer's slot inspector (renderInspector).
• Renders as a gated picker (checkbox + HTML color input) — same pattern
  as the existing Cards-view color params. Unchecked = no override
  emitted to the URL.
• Add `color` slot to hero (accent color override).

Out of v3.4.2

• Color slots on other cards (divider/neon/glitch/etc) — those cards
  aren't yet inline-edit-capable so no rush
• Inline color picker on the preview itself (v3.5+)