add vanilla DOM web backend

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+# DOM web backend
+
+The web backend keeps the existing Pardes core and replaces the old
+Emscripten/SDL/WebGL shell. Zig produces a `wasm32-freestanding` module with no
+imports. Vanilla JavaScript owns `requestAnimationFrame`, browser input, window
+sizing, and effect/IO dispatch; HTML and CSS render selectable, accessible text.
+There is no canvas.
+
+The default web build links Tree-sitter's C runtime and the Zig grammar inside
+that same import-free module. A small freestanding compatibility layer supplies
+the C ABI locally, so source panes retain the core's Tree-sitter highlighting
+without Emscripten, WASI, or browser libc imports. Other grammar tiers remain
+selectable with `-Dtree-sitter=minimal` or `-Dtree-sitter=full`.
+
+`src/web.zig` is the narrow WASM ABI. Events enter through exported functions,
+and `pardes_frame` writes the canonical `Surface` into a packed 20-byte cell
+array. `src/web/app.mjs` reads that array in one linear pass and patches stable
+DOM nodes. Core effects become browser operations (links, downloads, clipboard)
+or `pardes-io` custom events for a process-capable embedding host.
+
+Build a replay from a dump:
+
+```sh
+zig build web \
+  -Dplatform=web \
+  -Dtarget=wasm32-freestanding \
+  -Dtree-sitter=zig \
+  -Ddump=test/web-snapshots/source-list.dump.zon
+```
+
+The output is in `zig-out/web`. Serve that directory over HTTP; browsers do not
+allow a useful WASM module load from `file:` URLs.
+
+Two dependency-free test layers cover the boundary:
+
+```sh
+# Real WASM in Node plus a small DOM double.
+zig build web-harness \
+  -Dplatform=web \
+  -Dtarget=wasm32-freestanding \
+  -Dtree-sitter=zig \
+  -Ddump=test/web-snapshots/source-list.dump.zon
+
+# Real headless Chrome, DOM rendering, and browser touch input.
+zig build web-e2e
+```
+
+Set `PARDES_CHROME` when Chrome/Chromium is not on `PATH` or in a normal
+Playwright/Puppeteer cache.