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/tmux-manager.ts

@@ -2137,16 +2137,19 @@ export class TmuxManager extends EventEmitter implements TerminalMultiplexer {
       return false;
     }
 
-    try {
-      execSync(`${this.tmux()} resize-window -t ${shellescape(muxName)} -x ${cols} -y ${rows}`, {
-        timeout: EXEC_TIMEOUT_MS,
-        stdio: 'ignore',
-      });
-      return true;
-    } catch (err) {
-      console.error('[TmuxManager] Failed to resize tmux window:', err);
-      return false;
-    }
+    // Fire-and-forget: this runs on the interactive resize path (WS {t:'z'} and
+    // HTTP /resize), so use a non-blocking exec — a slow/hung tmux must not stall
+    // the Fastify event loop while other sessions' input/SSE are served. The sole
+    // caller (Session.resize) ignores the result, and under `window-size manual`
+    // the subsequent ptyProcess.resize is subordinate to this authoritative size.
+    exec(
+      `${this.tmux()} resize-window -t ${shellescape(muxName)} -x ${cols} -y ${rows}`,
+      { timeout: EXEC_TIMEOUT_MS },
+      (err) => {
+        if (err) console.error('[TmuxManager] Failed to resize tmux window:', err);
+      }
+    );
+    return true;
   }
 
   isAvailable(): boolean {