Confirm text-stage regeneration against unsaved edits with an inline diff

[atomantic]

Summary

Text-stage Generate was hard-disabled once the output editor went dirty ("Save or discard your edits first"), which turned "you might lose your edits" into "you can't generate at all" — with no way to see what the new version would have been. StageHistoryModal only diffs persisted runHistory, so an unsaved draft had nothing to compare against.

Generate is live again while the editor is dirty, behind a two-step flow in TextStagePanel.jsx:

1. Clicking Generate with unsaved output edits opens an inline confirm row ("Generate & compare" / "Cancel") instead of firing immediately — no alert/confirm, per the repo's confirmation-UX convention.
2. The result is held in the existing shared InlineDiff against the live editor text (not a kickoff snapshot, so typing while the review is open keeps the comparison honest) rather than being written into the textarea. Use new version applies it; Keep my edits leaves the text exactly as typed — still dirty against the newly persisted output, so Save pushes the user's version back over it.

Details worth calling out:

• The generated stage is lifted to the parent via onStageUpdate either way — the server already persisted the run, so runHistory/lastRunId stay truthful while the user decides.
• The parent-driven reset effect is suppressed only for the record under review (keyed on issue.id:stageId); switching issue or stage drops the pending comparison and reloads normally. serverGenerating still syncs unconditionally so a held review can't strand the spinner.
• Identical incoming text skips the review entirely — there is no decision to make.
• Save is disabled while a comparison is open, so a pending decision can't be sidestepped into a half-resolved state.
• Reuses the existing outputDirty signal and the existing InlineDiff component; no new diff rendering and no parallel dirty tracking.

Closes #3398

Test plan

• cd client && npm test — 530 files / 6095 tests pass.
• TextStagePanel.test.jsx gains a regenerate-over-unsaved-edits review (#3398) block covering: the diff appears and the editor keeps the unsaved text; applying replaces the editor text and dismisses the diff; canceling preserves the unsaved edits and re-enables Save; identical text skips the review; the stage is still lifted to the parent while the review is open.
• The pre-existing dirty-gate test was updated to the new behavior: Generate stays enabled, routes through the confirm row, and does not call the API until the user confirms.
• npx eslint clean on the touched files.

Review follow-ups (codex, 2 rounds → clean)

• The review decision runs at resolve time against the persisted text the run started from (via a draft ref), not a kickoff snapshot — so text typed while a multi-minute generate is in flight is reviewed too, not silently replaced.
• A pending review carries the record it belongs to and is gated both where it is set and where it renders; a result landing after the panel moved to another issue/stage is dropped, and it no longer calls onStageUpdate (which would patch whichever record is on screen now).
• Generate and "Generate & compare" both disable on generating / actionsGated / saving; the confirm row hides once the edits it warned about are gone. The save gate is reachable with a seed-only edit, since the dirty routing keys on output drift.
• Three added tests cover the in-flight edit, the stale-record drop, and the save gate; each was verified to fail with its guard removed.