refactor(term): document and test mode 2031 unsubscribe replay safety

Explain why the `CSI ?2031l` handler is intentionally not replay-guarded
(we only push CSI 997 on subscribe, so clearing stale bookkeeping during
replay is a no-op a later real `?2031h` restores) and extend the test to
actually exercise the replay path, not just the non-replay path.

src/renderer/src/components/terminal-pane/terminal-appearance.test.ts

@@ -245,6 +245,7 @@ describe('installMode2031Handlers', () => {
     // on subscribe, never on unsubscribe.
     const h = setup()
     try {
+      // Non-replay path: subscribe then unsubscribe clears state.
       await writeSync(h.term, '\x1b[?2031h')
       h.paneLastThemeMode.set(1, 'dark')
       expect(h.paneMode2031.get(1)).toBe(true)
@@ -252,6 +253,18 @@ describe('installMode2031Handlers', () => {
       await writeSync(h.term, '\x1b[?2031l')
       expect(h.paneMode2031.has(1)).toBe(false)
       expect(h.paneLastThemeMode.has(1)).toBe(false)
+
+      // Replay path: resubscribe, then receive `?2031l` during a replay
+      // window. The `l` handler must still clear — this is the invariant
+      // promised by the test name.
+      await writeSync(h.term, '\x1b[?2031h')
+      h.paneLastThemeMode.set(1, 'dark')
+      expect(h.paneMode2031.get(1)).toBe(true)
+
+      replayIntoTerminal(h.pane, h.replayingPanesRef, '\x1b[?2031l')
+      await new Promise<void>((resolve) => h.term.write('', resolve))
+      expect(h.paneMode2031.has(1)).toBe(false)
+      expect(h.paneLastThemeMode.has(1)).toBe(false)
     } finally {
       h.dispose()
     }

src/renderer/src/components/terminal-pane/terminal-appearance.ts

@@ -86,6 +86,10 @@ export function installMode2031Handlers(deps: Mode2031HandlerDeps): IDisposable[
       }
       return false
     }),
+    // Why no replay guard on the unsubscribe branch: clearing stale bookkeeping
+    // is harmless. We only push CSI 997 on subscribe, never on unsubscribe, so
+    // even if a cold-restore replay carries `?2031l`, this handler just deletes
+    // map entries that a later real `?2031h` will re-populate normally.
     deps.parser.registerCsiHandler({ prefix: '?', final: 'l' }, (params) => {
       if (hasMode2031(params)) {
         deps.paneMode2031.delete(deps.paneId)