Replace the naive RGB lightness-shift color system with a contrast-aware
pipeline that derives two purpose-specific colors per collaborator from
seed colors in the palette:

- strokeColor: meets 3.1:1 contrast against white (WCAG 2.1 SC 1.4.11
  for UI components like borders and block highlights)
- backgroundColor: meets 7:1 contrast against white (WCAG 2.1 SC 1.4.3
  AA for white text on colored cursor labels)

Colors are manipulated in OKLCH space (Björn Ottosson's OKLAB) for
perceptually uniform darkening — only lightness is reduced while chroma
and hue are preserved. This avoids the hue drift that CIE LCH exhibits
when darkening blues. The implementation is ~60 lines of pure math with
zero external dependencies.

Also updates the seed palette to use more vibrant, distinct colors and
replaces the RGB-shift overflow strategy with OKLCH hue rotation for
generating new colors when all palette slots are taken.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Replace the inline box-shadow approach for RTC block highlighting with
the same outline-based method core uses for `.is-highlighted` blocks.
This ensures consistent outline style, width, offset, and zoom scaling
across both the default selection and collaborator highlights.

- Switch from inline `style.boxShadow` to a `.is-collaborator-selected`
  CSS class with a `--collaborator-outline-color` custom property
- Reuse the `selected-block-focus()` mixin from `@wordpress/base-styles`
- Add `:not(:focus)` guard so the user's own selection always wins
- Extract shared `collaborator-contrast-shadow()` mixin used by both the
  block highlight and avatar components
- Add compiled `COLLABORATOR_CONTRAST_SHADOW` constant for iframe styles

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Redesign the cursor label from a plain text tag to a pill-shaped badge
with the collaborator's avatar. The label defaults to showing just the
avatar circle and expands on hover to reveal the username.

- Add `collaborator-avatar()` mixin for shared avatar styles between
  the presence list and cursor label, using `--avatar-url` CSS custom
  property
- Restyle cursor label as a flex pill with avatar + name, white outline
  contrast ring, and hover expand transition
- Add white outline + elevation shadow to cursor caret for contrast
- Fix block highlight inset shadow: use a wider inset that extends past
  the outline so the white inner ring remains visible
- Consolidate `collaborator-contrast-shadow()` to include inset ring,
  fixing the `--with-color-border` avatar variant losing its inner ring
- Sync all changes to `overlay-iframe-styles.ts` for iframe injection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>