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/lib/components/Editor.svelte

@@ -128,10 +128,12 @@
     return () => view?.destroy();
   });
 
-  // When a different diagram is loaded, replace the whole document. Depend only
-  // on activeId; read content untracked so typing doesn't reset the editor.
+  // Replace the whole document when a different diagram is loaded (activeId) or
+  // when content is swapped programmatically (revision, e.g. an applied
+  // optimization). Read content untracked so plain typing never resets it.
   $effect(() => {
-    void store.activeId; // re-run only when the selected diagram changes
+    void store.activeId; // re-run when the selected diagram changes…
+    void store.revision; // …or when an optimization rewrites the source.
     if (!view) return;
     untrack(() => {
       const incoming = store.content;