Add M4 orchestration: broadcast, idle notifications, palette

tree.ts
- LeafNode gains broadcast?: boolean
- walkLeaves(root) generator; toggleBroadcast helper

ops.ts (PaneOps)
- toggleBroadcast, broadcastFrom, setActivePane, registerPaneId,
  notify; activeLeafId data field.

XtermPane.svelte
- onSpawn(paneId), onInput(b64), onDataReceived(),
  and focusTrigger prop. All optional; backward-compatible.

LeafPane.svelte
- 📡 broadcast toggle; 5s idle detection -> ops.notify (once per
  idle cycle); active + broadcasting border colors; click-to-focus
  via setActivePane + focusTrigger bump.

New Notifications.svelte
- Top-right toast stack, slide-in, 5s auto-dismiss + click ×.

New Palette.svelte
- Modal overlay, backdrop, filtered leaf list with ↑/↓ + Enter,
  Escape to close.

App.svelte
- paneIdByLeaf Map for routing; notifications array + auto-dismiss;
  activeLeafId; Ctrl+K global listener; broadcastFrom routes via
  walkLeaves + writeToPane to all other broadcast leaves; ⌘K button
  in titlebar.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/Notifications.svelte

@@ -0,0 +1,81 @@
+<script lang="ts">
+  export interface Toast {
+    id: number;
+    message: string;
+  }
+
+  let {
+    notifications,
+    onDismiss,
+  }: {
+    notifications: Toast[];
+    onDismiss: (id: number) => void;
+  } = $props();
+</script>
+
+<div class="toast-stack">
+  {#each notifications as t (t.id)}
+    <div class="toast">
+      <span class="toast-msg">{t.message}</span>
+      <button
+        class="toast-x"
+        onclick={() => onDismiss(t.id)}
+        aria-label="Dismiss notification"
+      >×</button>
+    </div>
+  {/each}
+</div>
+
+<style>
+  .toast-stack {
+    position: fixed;
+    top: 36px;
+    right: 12px;
+    display: flex;
+    flex-direction: column;
+    gap: 6px;
+    z-index: 100;
+    pointer-events: none;
+  }
+  .toast {
+    pointer-events: auto;
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    min-width: 220px;
+    max-width: 320px;
+    padding: 8px 10px 8px 14px;
+    background: #1f1f1f;
+    color: #ddd;
+    border: 1px solid #3a5a8c;
+    border-left-width: 3px;
+    border-radius: 4px;
+    font-size: 12px;
+    box-shadow: 0 6px 18px rgba(0, 0, 0, 0.45);
+    animation: slide-in 180ms ease-out;
+  }
+  .toast-msg {
+    flex: 1 1 auto;
+    line-height: 1.3;
+    word-break: break-word;
+  }
+  .toast-x {
+    flex: 0 0 auto;
+    background: transparent;
+    border: none;
+    color: #777;
+    font-size: 16px;
+    line-height: 1;
+    padding: 2px 6px;
+    cursor: pointer;
+    border-radius: 3px;
+  }
+  .toast-x:hover {
+    background: #2a2a2a;
+    color: #ddd;
+  }
+  @keyframes slide-in {
+    from { transform: translateX(20px); opacity: 0; }
+    to { transform: translateX(0); opacity: 1; }
+  }
+</style>