fix: dedupe run progress replay-gap notices

[alectimison-maker]

Summary

• replace the replay-gap boundary high-water mark with a once-per-run UI marker
• keep restored messages with the legacy numeric marker deduplicated
• cover advancing acknowledgement boundaries in the Chrome/Firefox regression suite

Motivation

When another panel acknowledges events while a displayed run remains behind, ackedSeq can advance across state polls while lastRenderedSeq stays unchanged. The previous boundary-based marker treated every larger boundary as a new gap and appended the same notice repeatedly.

Closes #2750.

Design

The marker now records whether the run's assistant message has shown a replay-gap notice, instead of recording the largest unavailable sequence. The rendered-event cursor is intentionally unchanged so acknowledged terminal snapshots can still restore finalContent. Restored HTML carrying the previous data-replay-gap-before-seq marker is treated as already notified.

Chrome and Firefox use the same implementation.

Testing

• npm test — pass
  • rich-text toolbar guard: 33 passed
  • core suite: 1623 passed, 0 failed
  • security corpus: 60/60 passed
• npm run test:webmcp — both functional smoke phases pass locally; the process then times out in CDP cleanup. The identical cleanup timeout reproduces on an unmodified upstream/main@41941bf3 worktree.

Compatibility and risks

Low risk. This is UI-only state on the existing assistant message element. It does not change the run journal, acknowledgement behavior, protocol, model prompts, permissions, or persisted data schema.

Scope

Changed only the mirrored Chrome/Firefox sidepanel replay-gap branch and its existing regression tests.

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[Copilot]

Pull request overview

Prevents duplicate replay-gap notices as acknowledgement boundaries advance, while preserving legacy marker compatibility.

Changes:

• Uses a once-per-run replay-gap marker in both sidepanels.
• Preserves deduplication for restored legacy markers.
• Adds cross-browser regression coverage for advancing boundaries.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/run.js	Updates and extends replay-gap regression tests.
src/firefox/src/ui/sidepanel.js	Implements Firefox notice deduplication.
src/chrome/src/ui/sidepanel.js	Implements Chrome notice deduplication.

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