feat(optimize): Diagram Doctor panel to declutter tangled flowcharts

Add an Optimize side panel that analyses the active flowchart and applies
one-click, reversible source rewrites to make dense graphs readable.

- optimize.ts: pure, label-aware flowchart parser + metrics, a 0-100
  readability score, and transforms — ELK layered layout, node/rank
  spacing + curved edges, de-emphasising cross-cutting hub edges
  (event bus / audit / cost-style fan-ins), duplicate-edge removal,
  direction toggle, and a non-destructive focus variant.
- mermaid.ts: register @mermaid-js/layout-elk so `layout: elk` renders.
- OptimizePanel.svelte + Toolbar toggle; panels are mutually exclusive.
- Preview: render the focus variant when a node is spotlighted, with a
  clear-focus pill.
- store/Editor: applySource() bumps a revision so programmatic rewrites
  re-sync CodeMirror and stay undoable with the editor's history.

src/App.svelte

@@ -7,6 +7,7 @@
   import Editor from './lib/components/Editor.svelte';
   import Preview from './lib/components/Preview.svelte';
   import GitPanel from './lib/components/GitPanel.svelte';
+  import OptimizePanel from './lib/components/OptimizePanel.svelte';
   import Toasts from './lib/components/Toasts.svelte';
 
   // Editor/preview split ratio (fraction taken by the editor).
@@ -112,6 +113,9 @@
           <div class="faint">Pick one from the sidebar, or create a new diagram with ＋.</div>
         </div>
       {/if}
+      {#if store.showOptimize}
+        <OptimizePanel />
+      {/if}
       {#if store.showGit}
         <GitPanel />
       {/if}