Add WASM webapp migration plan

PLAN.md

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+# TrueScad WASM Webapp Plan
+
+Convert TrueScad from a GTK desktop app to a WASM webapp hosted on GitHub Pages.
+The GTK desktop app is being abandoned in favor of a web-first approach.
+
+## Architecture
+
+- **Rust** compiles to a `cdylib` WASM module via `wasm-pack`, exposing a `wasm-bindgen` API
+- **Vanilla JS** + HTML/CSS handles all UI
+- **CodeMirror 6** (bundled via npm/esbuild) for the Lua editor
+- **`<canvas>`** receives pixel data via `putImageData` directly from the Rust renderer
+- Rendering is **on-demand**: triggered by Run and drag events only
+
+## Rust WASM API
+
+```rust
+#[wasm_bindgen] pub fn eval(code: &str) -> EvalResult
+#[wasm_bindgen] pub fn render(width: u32, height: u32) -> Uint8ClampedArray
+#[wasm_bindgen] pub fn rotate(dx: f64, dy: f64)
+#[wasm_bindgen] pub fn pan(dx: f64, dy: f64)
+#[wasm_bindgen] pub fn export_stl() -> Uint8Array
+```
+
+`Uint8ClampedArray` slots directly into `new ImageData(arr, w, h)` → `ctx.putImageData()` with no copy.
+
+## Crate changes
+
+| Item | Action |
+|---|---|
+| `src/window.rs`, `editor.rs`, `menu.rs`, `object_widget.rs`, `settings.rs`, `mesh_view.rs` | Delete |
+| `src/render.rs`, `src/lib.rs` | Keep — pure math, WASM-ready |
+| `src/main.rs` | Replace with `wasm-bindgen` entry point |
+| Root `Cargo.toml` | Remove gtk/sourceview4/gdk/glib/cairo-rs/kiss3d/rayon/dirs/pollster; add `wasm-bindgen`, `web-sys` |
+| `kiss3ddeps/` | Delete (only existed for kiss3d) |
+| `luascad/` | Keep as-is (piccolo — WASM-ready) |
+
+## Frontend layout
+
+```
+index.html
+main.js          ← wires CM6, canvas drag, buttons → WASM calls
+main.css
+pkg/             ← wasm-pack output (truescad.wasm + JS bindings)
+node_modules/    ← CM6 (dev only, bundled by esbuild)
+```
+
+## Phase order
+
+1. **Strip root crate** — delete GTK files, rewrite `Cargo.toml`
+2. **`wasm-bindgen` API** — implement `src/lib.rs` wrapping `render.rs` + `luascad::eval`
+3. **Verify WASM builds** — `wasm-pack build --target web`
+4. **Frontend scaffold** — `index.html` + `main.js` + CodeMirror 6
+5. **Wire JS ↔ WASM** — Run button, drag-to-rotate, STL export download
+6. **GitHub Actions** — `wasm-pack build --release` + deploy `dist/` to `gh-pages`
+
+## Status
+
+- [x] piccolo migration complete (`luascad` uses piccolo 0.3, no native C deps)
+- [ ] Phase 1: Strip root crate
+- [ ] Phase 2: wasm-bindgen API
+- [ ] Phase 3: Verify WASM builds
+- [ ] Phase 4: Frontend scaffold
+- [ ] Phase 5: Wire JS ↔ WASM
+- [ ] Phase 6: GitHub Actions + GitHub Pages