web: add standalone web worker example

[Fristender]

This requires #961 to be merged first.

Adds a third web target, web-standalone, that runs the whole dvui app inside a Web Worker: WASM + WebGL on an OffscreenCanvas, events delivered via SharedArrayBuffer + Atomics, frames returned to the main thread as transferred ImageBitmaps. Includes JSPI support so the blocking event loop doesn't starve the worker event loop.

Review guide

• web-common.js:96-128 — shared-memory protocol constants (signal flag, cursors, canvas info, 256-slot event ring, 4 KiB string area). Contract between the two new files; check offsets add up (TOTAL_SHARED_SIZE = 9472).
• web-standalone.js:33-77 — dvuiStandalone() entry: canvas lookup, dvui_debug flag, SharedArrayBuffer/Atomics/COOP-COEP preflight checks.
• web-standalone.js:105-148 — pushEvent/pushString: ring-full drop policy, 32-bit wrap tricks, string-area wraparound (best-effort, no synchronization with worker reads — worth a look).
• web-standalone.js:158-261 — DOM listeners mirroring web.js (mouse/wheel/key/IME/touch), each pushing ring events.
• web-standalone.js:264-318 — messages from worker: bitmap, cursor, text_input, open_url, download, clipboard_set, panic.
• web-worker.js:43-119 — JSPI detection, suspending wasmWaitEventJspi/wasmSleepJspi, MessageChannel macrotask yield, Edge STATUS_ACCESS_VIOLATION rationale comment.
• web-worker.js:153-186 — drainEvents(): key/text events read strings from the shared string area and re-enter wasm via add_event.
• web-worker.js:201-260 — init: OffscreenCanvas creation, wasm instantiate, main() via WebAssembly.promising when JSPI is available.
• web.zig:42-45 and web.zig:804-836 — new wasm_wait_event extern, InitOptions/initWindow(), waitEventTimeout(), and renderPresent() now shipping the bitmap.
• web-standalone.zig:30-46 — SDL-standalone-shaped main(): init → frame → waitEventTimeout loop.
• build.zig:998-1006, build.zig:1036-1058, build.zig:1657-1704 — separate export-symbol list, separate backend module, and addWebStandaloneExample().

Known gaps vs. web-app (no feature parity yet)

1. File dialog / file picker — worker inherits the no-op stubs (web-common.js:1073-1081); web-app has the real <input type="file"> + filesCache implementation (web.js:213-265). dvui.dialogs file open is a silent no-op in standalone.
2. Clipboard copy fallback — standalone only uses navigator.clipboard.writeText (web-standalone.js:299-303); web-app falls back to hidden-input execCommand("copy") for non-secure contexts (web.js:267-283).
3. prefers-reduced-motion — hardcoded to "no preference" (web-worker.js:330-332); web-app queries matchMedia (web.js:199-211). Not yet forwarded through the shared buffer.
4. wasm_refresh — no-op in worker mode (web-worker.js:266-268); web-app uses it to schedule requestAnimationFrame renders (web.js:329-332). Async operations that expect to trigger a repaint won't.
5. Window options ignored — initWindow() discards size/min/max/vsync/title/icon (web.zig:822-825); size comes from CSS, title/icon from the HTML.
6. platform string — sent in the init message (web-standalone.js:325) but never consumed in web-worker.js.
7. No quit path — the example's main_loop runs forever (web-standalone.zig:38-45); nothing maps to "window closed".
8. Panic UX — standalone alert()s (web-standalone.js:304-307) vs. console-only in web-app.

[MichaelBelousov]

@Fristender is this PR worth renaming to like implement part of a web worker example? I feel like some of the missing features are worth emphasizing that it isn't done?