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-lock.json

@@ -15,6 +15,7 @@
         "@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",
@@ -625,6 +626,19 @@
       "integrity": "sha512-l0h88YhZFyKdXIFNfSWpyjStDjGHwZ/U7iobcK1cQQD8sejsONdQtTVU+1wVN1PBw40PiiHB1vA5S7VTfQiP9g==",
       "license": "MIT"
     },
+    "node_modules/@mermaid-js/layout-elk": {
+      "version": "0.2.1",
+      "resolved": "https://registry.npmjs.org/@mermaid-js/layout-elk/-/layout-elk-0.2.1.tgz",
+      "integrity": "sha512-MX9jwhMyd5zDcFsYcl3duDUkKhjVRUCGEQrdCeNV5hCIR6+3FuDDbRbFmvVbAu15K1+juzsYGG+K8MDvCY1Amg==",
+      "license": "MIT",
+      "dependencies": {
+        "d3": "^7.9.0",
+        "elkjs": "^0.9.3"
+      },
+      "peerDependencies": {
+        "mermaid": "^11.0.2"
+      }
+    },
     "node_modules/@mermaid-js/parser": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/@mermaid-js/parser/-/parser-1.1.1.tgz",
@@ -2216,6 +2230,12 @@
         "@types/trusted-types": "^2.0.7"
       }
     },
+    "node_modules/elkjs": {
+      "version": "0.9.3",
+      "resolved": "https://registry.npmjs.org/elkjs/-/elkjs-0.9.3.tgz",
+      "integrity": "sha512-f/ZeWvW/BCXbhGEf1Ujp29EASo/lk1FDnETgNKwJrsVvGZhUWCZyg3xLJjAsxfOmt8KjswHmI5EwCQcPMpOYhQ==",
+      "license": "EPL-2.0"
+    },
     "node_modules/es-toolkit": {
       "version": "1.46.1",
       "resolved": "https://registry.npmjs.org/es-toolkit/-/es-toolkit-1.46.1.tgz",