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.

package.json

@@ -20,6 +20,7 @@
     "@codemirror/state": "^6.4.1",
     "@codemirror/view": "^6.28.0",
     "@lezer/highlight": "^1.2.0",
+    "@mermaid-js/layout-elk": "^0.2.1",
     "@tauri-apps/api": "^2.0.0",
     "@tauri-apps/plugin-dialog": "^2.0.0",
     "codemirror": "^6.0.1",