fix: layout-engine comment/track highlight config

[financialvice]

• Respect comments highlight config (incl. active) in layout-engine renderer and expose CSS var fallbacks
• Add CSS var support for track-change highlights in layout-engine
• Add overlay highlight config + CSS vars; sync active comment highlight updates; docs/types update

Tests:
pnpm --filter @superdoc/painter-dom test (fails in existing table/border-utils.test.ts, 2 failures)
pnpm --filter @superdoc/super-editor test
pnpm --filter superdoc test

Visual:

• Dev app w/ layout engine ON; verified CSS var overrides for comment highlight, track-change highlight, and overlay highlight

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve per-comment internal/external flags in refresh

The new refreshCommentHighlights rebuilds the comments array from data-comment-ids using a single isInternal flag derived from data-comment-internal, which is set to true if any comment on the span is internal. If a span has mixed internal/external comments (supported by storing multiple IDs), switching the active thread to an external comment will still be treated as internal during refresh and will apply internal highlight colors/overrides. This regression only shows up on active-thread updates because the initial paint uses the real comment metadata. Consider persisting per-comment internal state (e.g., encode with the IDs) or reusing the original run metadata so refresh can choose the correct color.

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