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.

README.md

@@ -20,6 +20,14 @@ TypeScript** frontend using **CodeMirror 6** and **Mermaid 11**.
 - **Git version control built in** — view the working-tree status, write commit
   messages, browse history, create branches and switch between them. Each branch
   keeps its own set of diagrams, just like normal Git.
+- **Optimize panel (Diagram Doctor)** — analyse a tangled flowchart and declutter
+  it in one click: a readability score and metrics (hubs, density, cross-group
+  edges), plus source rewrites for the **ELK layered layout**, extra
+  spacing/curved edges, **de-emphasising cross-cutting hub edges** (event bus,
+  audit, cost-style fan-ins), and removing duplicate edges. A non-destructive
+  **focus mode** spotlights any node and its neighbours so you can read a dense
+  graph without changing it. Every rewrite lands in the editor and is reversible
+  with ⌘Z.
 - **Export** the current diagram to **SVG** or **PNG** (2× scale).
 - **Project registry** — recently opened projects are remembered in a local
   SQLite database so you can jump back in from the start screen.
@@ -35,9 +43,10 @@ Mermix/
 │  ├─ lib/
 │  │  ├─ api.ts             # typed wrappers over Tauri commands
 │  │  ├─ store.svelte.ts    # central rune-based app state
-│  │  ├─ mermaid.ts         # render + error capture
+│  │  ├─ mermaid.ts         # render + error capture (registers ELK layout)
+│  │  ├─ optimize.ts        # flowchart analysis + declutter transforms (pure)
 │  │  ├─ export.ts          # SVG / PNG export via save dialog
-│  │  └─ components/        # Sidebar, Editor, Preview, GitPanel, Toolbar, …
+│  │  └─ components/        # Sidebar, Editor, Preview, GitPanel, OptimizePanel, …
 │  └─ main.ts
 └─ src-tauri/               # Rust backend
    ├─ src/