User event
    │
    ▼
Tool (onMouseDown / onMouseMove / onMouseUp)
    │  dispatches Command
    ▼
History.dispatch(command)
    │  calls
    ▼
reducer(Scene, Command) → Scene   ← pure function, no side effects
    │  notifies
    ▼
Subscribers (toolbar, properties panel, canvas view)
    │
    ▼
render(ctx, scene, canvas)        ← called every requestAnimationFrame

src/
├── core/
│   ├── scene.ts            # Scene interface + factory
│   ├── viewport.ts         # Pan/zoom math (screenToWorld, worldToScreen)
│   └── app_state.ts        # AppState (activeTool, colors, grid, …)
├── elements/
│   └── element.ts          # Discriminated union for all element types
├── commands/
│   └── commands.ts         # Full Command discriminated union
├── engine/
│   ├── reducer.ts          # Pure (Scene, Command) → Scene
│   └── history.ts          # Undo/redo stack + pub/sub
├── io/
│   ├── markasso.ts         # .markasso save / load (exportMarkasso, importMarkasso)
│   ├── mermaid.ts          # Mermaid parser + layout engine → Markasso elements
│   ├── session.ts          # localStorage auto-save / restore + quota warning toast
│   └── share.ts            # URL-hash scene encoding / decoding for share links
├── rendering/
│   ├── renderer.ts         # rAF render loop entry point
│   ├── draw_element.ts     # Per-type draw dispatch (with rotation + shape labels)
│   ├── draw_grid.ts        # Dot / line / mm graph-paper grids
│   ├── draw_selection.ts   # Selection box, resize/rotation/endpoint handles + hit testing
│   └── export.ts           # exportPNG / exportSVG / exportHTML (bounding-box auto-fit)
├── tools/
│   ├── tool.ts             # Tool interface
│   ├── select_tool.ts      # Hit test, marquee, drag-move, resize
│   ├── rectangle_tool.ts
│   ├── ellipse_tool.ts
│   ├── rombo_tool.ts       # Rhombus tool
│   ├── line_tool.ts        # Line tool; arrowheads are a style on line elements
│   ├── pen_tool.ts         # Freehand with Catmull-Rom smoothing + stylus pressure
│   ├── text_tool.ts        # Invisible overlay textarea + in-place editing
│   └── sticky_tool.ts      # Sticky note with editable text and color picker
└── ui/
    ├── canvas_view.ts       # DOM event hub → world coords → tool
    ├── toolbar.ts           # Floating islands: tools · undo/zoom · import · export · share
    ├── context_panel.ts     # Properties panel (style, formatting, shadow, alignment)
    ├── command_palette.ts   # Ctrl+K fuzzy-search command launcher
    ├── element_search.ts    # Ctrl+F canvas element search overlay
    ├── minimap.ts           # Collapsible canvas overview + viewport panning
    ├── image_import.ts      # File picker, drag-and-drop, paste — images + .markasso
    ├── settings.ts          # Hamburger panel: grid, accent color, theme, language
    ├── mobile_action_bar.ts # Compact bottom-right action bar for touch devices
    └── shortcuts.ts         # Global keyboard map