fix(terminal): address re-review findings on the pane-buffer rework

Follow-up to the PR #112 re-review (all six prior blockers were already
resolved; these are new issues the rework introduced):

- app.js: define the missing `_scheduleTerminalRepaint()` helper. It was
  called from both WebGL-fallback paths (onContextLoss + long-task trip)
  but defined nowhere, so each fallback threw `TypeError` and lost the
  post-fallback repaint, leaving a stale/blank terminal. Implemented as an
  rAF-debounced full refresh (matches the old inline `terminal.refresh`).
- app.js: clear terminal load-state on the two post-write stale-select
  early-returns (cached-buffer + rewrite branches), matching the other
  four checks. Switching away from a mid-loading tab no longer leaks a
  permanent `.tab-loading` spinner / `aria-busy=true`.
- terminal-ui.js + app.js: gate the post-resize TUI-redraw settle on an
  actual dimension change. `sendResize` now returns whether dims changed;
  a same-size tab switch sends no SIGWINCH, so the wait is skipped instead
  of charging a flat tax on every non-shell switch. Literal hoisted to
  `TUI_REDRAW_SETTLE_MS`.
- tmux-manager.ts: `resizeWindow()` uses a non-blocking `exec` instead of
  `execSync` so the interactive WS/HTTP resize path can't stall the
  Fastify event loop on a slow/hung tmux. Sole caller already fire-and-
  forgets the result; test updated to assert the async dispatch.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -702,6 +702,22 @@ class CodemanApp {
     this._webglLongTaskObserver = null;
   }
 
+  /**
+   * Repaint the full terminal viewport after a renderer swap (WebGL → canvas/DOM).
+   * Scheduled on the next frame so it lands after the addon teardown settles, and
+   * debounced so the context-loss and long-task fallback paths can't double-fire.
+   * No-ops safely if the terminal isn't ready.
+   */
+  _scheduleTerminalRepaint() {
+    if (this._terminalRepaintScheduled) return;
+    this._terminalRepaintScheduled = true;
+    const raf = typeof requestAnimationFrame === 'function' ? requestAnimationFrame : (cb) => setTimeout(cb, 0);
+    raf(() => {
+      this._terminalRepaintScheduled = false;
+      try { this.terminal?.refresh(0, this.terminal.rows - 1); } catch {}
+    });
+  }
+
   _disableWebGLSticky(reason) {
     try {
       localStorage.setItem('codeman-webgl-disabled', JSON.stringify({ reason, at: Date.now() }));
@@ -3008,7 +3024,7 @@ class CodemanApp {
       // a small region with empty rows below the status bar.
       // sendResize is a no-op on the server when dims haven't changed, so
       // calling it every tab switch is cheap.
-      await this.sendResize(sessionId, { forceHttp: true }).catch(() => {});
+      const dimsChanged = await this.sendResize(sessionId, { forceHttp: true }).catch(() => false);
       if (this._isStaleSelect(selectGen)) {
         this._clearTerminalLoadState(sessionId, selectGen);
         return;
@@ -3027,7 +3043,10 @@ class CodemanApp {
         this._setTerminalLoadState(sessionId, selectGen, 'replaying');
         this._resetTerminalForReplay();
         await this.chunkedTerminalWrite(cachedBuffer, TERMINAL_CHUNK_SIZE, bufferLoadOwner);
-        if (this._isStaleSelect(selectGen)) return;
+        if (this._isStaleSelect(selectGen)) {
+          this._clearTerminalLoadState(sessionId, selectGen);
+          return;
+        }
         this.terminal.scrollToBottom();
         _crashDiag.log('CACHE_DONE');
       } else if (sessionIsBusy) {
@@ -3037,11 +3056,12 @@ class CodemanApp {
       }
 
       // Give TUI sessions a short chance to redraw after resize before the
-      // fresh buffer fetch captures the live mux pane. Shell sessions and
-      // snapshot restores do not need this delay, so terminal content can
-      // appear immediately when switching between shells.
-      if (session?.mode !== 'shell') {
-        await new Promise((resolve) => setTimeout(resolve, 400));
+      // fresh buffer fetch. Only needed when the resize actually changed
+      // dimensions (a real SIGWINCH → Ink redraw); a same-size tab switch sent
+      // no resize, so waiting would just add latency. Shell sessions never need
+      // it, so terminal content can appear immediately when switching shells.
+      if (session?.mode !== 'shell' && dimsChanged) {
+        await new Promise((resolve) => setTimeout(resolve, TUI_REDRAW_SETTLE_MS));
         if (this._isStaleSelect(selectGen)) {
           this._clearTerminalLoadState(sessionId, selectGen);
           return;
@@ -3075,7 +3095,10 @@ class CodemanApp {
           }
           // Use chunked write for large buffers to avoid UI jank
           await this.chunkedTerminalWrite(data.terminalBuffer, TERMINAL_CHUNK_SIZE, bufferLoadOwner);
-          if (this._isStaleSelect(selectGen)) return;
+          if (this._isStaleSelect(selectGen)) {
+            this._clearTerminalLoadState(sessionId, selectGen);
+            return;
+          }
           // Ensure terminal is scrolled to bottom after buffer load
           this.terminal.scrollToBottom();
         }