Fix M4 reactivity bugs via context + class store

Symptoms in v0.1.0 install: 📡 broadcast button didn't change color
on toggle, × close button didn't remove the pane, blue active
border stuck on the first pane. All three were UI-not-rerendering-
on-state-change manifestations of the same prop-reactivity quirk
that drilling activeLeafId tried (and apparently failed) to fix.

Refactor to the Svelte 5 canonical pattern for shared reactive
state:

- New src/lib/layout/orchestration.svelte.ts with an Orchestration
  class. Reactive fields (activeLeafId, notifications, distros) are
  class-field $state declarations; methods mutate them directly.
  Provided via context (provideOrchestration / useOrchestration);
  no prop drilling.
- App.svelte: provideOrchestration(treeOps). Tree mutations remain
  closures over the App-level tree $state; the class delegates to
  them. Pane only takes `node` now.
- Pane.svelte / SplitNode.svelte: stop drilling ops + activeLeafId.
  Pure pass-through of node.
- LeafPane.svelte: useOrchestration(); `active = $derived(
  orch.activeLeafId === leaf.id)` reads the class field directly so
  Svelte 5 tracks it per-property.
- Notifications.svelte: receives notifications + onDismiss from App
  (which gets them from orch).
- Deleted src/lib/layout/ops.ts (TreeOps moved into orchestration).

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

src/lib/layout/Pane.svelte

@@ -1,24 +1,15 @@
 <script lang="ts">
   import type { TreeNode } from "./tree";
-  import type { PaneOps } from "./ops";
   import SplitNode from "./SplitNode.svelte";
   import LeafPane from "./LeafPane.svelte";
 
-  let {
-    node,
-    ops,
-    activeLeafId,
-  }: {
-    node: TreeNode;
-    ops: PaneOps;
-    activeLeafId: string | null;
-  } = $props();
+  let { node }: { node: TreeNode } = $props();
 </script>
 
 {#if node.kind === "split"}
-  <SplitNode {node} {ops} {activeLeafId} />
+  <SplitNode {node} />
 {:else}
   {#key node.id}
-    <LeafPane leaf={node} {ops} {activeLeafId} />
+    <LeafPane leaf={node} />
   {/key}
 {/if}