fix: live updates for rich-text and numeric merge values; internal editor accessors

[dazzatronus]

Two live-update fixes and a set of @internal editor accessors.

Fixes

• Rich-text updates on seek and during playback. A rich-text clip previously showed the first frame it rendered and never changed as the playhead moved; it now repaints each frame, so animated text and post-seek frames are correct.
• Numeric-looking merge values stay strings on text/html assets. A value like "3" on a text field was being coerced to a number, which a text/html asset can't render, so the clip silently stopped updating. Text/html content now keeps its string form (numeric coercion still applies to numeric properties).

Internal accessors (@internal, not public API)

Adds accessors for anchoring external UI to the canvas: a clip's on-screen rect, the backing <canvas> element, the clip a merge field drives, and a wait-until-assets-settled promise. All are @internal and stripped from the published type declarations — no change to the public API surface.

Verify

tsc clean; rich-text and merge-field Jest suites pass, with added coverage for the texture re-upload and the string-preservation case. Rich-text behaviour verified live across seek, playback, and live merge-field edits.

Release note

On squash-merge, the merge message drives the version — the public-facing changes here are bug fixes.

[github-actions]

🎉 This PR is included in version 2.14.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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