Offer an InlineDiff confirm before regenerating over unsaved stage edits

[atomantic]

Problem

Once Generate is dirty-gated (shipping in better/bugs-perf), there's still no lightweight way to compare an incoming regeneration against unsaved edits — StageHistoryModal only diffs persisted runHistory.

Decision

Add a "view diff before replacing" confirm affordance in client/src/components/pipeline/stages/TextStagePanel.jsx using the existing InlineDiff component (already used in StageHistoryModal.jsx:140): when the user chooses to generate with unsaved edits, show the diff of current unsaved text vs the incoming result before committing it into the editor.

Acceptance

• Diff view reachable from the generate flow when edits exist; accepting applies, canceling preserves edits

[atomantic]

Implementation note (unattended run): the issue says the affordance appears "when the user chooses to generate with unsaved edits," but the shipped dirty gate disabled Generate outright, so there was no such choice. Read as: the diff review is the escape hatch from the gate. Generate stays enabled while dirty and routes through an inline confirm row ("Generate & compare" / "Cancel"); the result is then held in InlineDiff against the live editor text until the user applies it or keeps their edits. The no-silent-clobber intent of the gate is preserved — nothing replaces the editor text without the user seeing the diff first. PR #3424.