v5.0.0: protocol v5 and the browser frontend

Protocol v5 replaces newline lines with bounded [u32 len][text] frames and REQ/RES/EVT envelopes. Requests carry correlation ids and the server replays the last 256 replies per connection, so a gateway retry cannot double-debit and a late reply cannot clobber a newer request. HELLO/PONG carry server time; crash rounds announce started_at and push coalesced 10 Hz ticks. Crash now commits sha256(seed) before a round and reveals the seed when it ends, so clients recompute the crash point.

Adds the webd/ gateway: one blocking thread, one ClientCore and one pinned upstream TLS connection per session, an HttpOnly session cookie, takeover, and a 60-second grace window. The browser sends Command JSON and renders View JSON; it never speaks the game protocol or stores credentials.

Adds the vanilla TypeScript browser app at /play/ (login, lobby, all four games, history, notices, crash verification), the nginx /play and /ws routes, and the headless DOM smoke test. webd and the app stay out of the TUI/GUI/server build graph.

The client/server version moves to 5.0.0 in lockstep; v4 peers are refused at HELLO.

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diff --git a/web/play/README.md b/web/play/README.md
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+# `web/play` — browser frontend
+
+Vanilla TypeScript + esbuild. The browser sends `Command` JSON and renders
+`View` JSON; it contains no game rules, no payout math and no wire protocol.
+The gateway in `webd/` owns the upstream TLS connection.
+
+## Build
+
+```sh
+npm ci
+npm run build      # writes app.js
+npm run typecheck
+npm test           # headless DOM smoke test (jsdom)
+```
+
+Commit the generated `app.js` so the site deploys without Node.
+
+## Local development
+
+Serve the repository's `web/` directory (for `/play/`) and put the gateway
+behind the same origin at `/ws`. The simplest local setup is nginx with the
+`web/nginx.conf.example` block pointed at a staging root and
+`ZAGAN_WEB_ORIGIN=http://127.0.0.1:8899`. For the app alone:
+
+```sh
+python3 -m http.server 8900 -d web/play
+```
+
+It will load, but the WebSocket will only connect through nginx or another
+proxy that routes `/ws` to `webd`.
+
+## Screens
+
+- Auth: login/register. The password is hashed with Web Crypto (`SHA-256`)
+  before it leaves the page; the gateway and browser never store it.
+- Lobby: four games, per-game bet, history.
+- Slots: five reels, win cells and paylines.
+- Roulette: canvas felt with straight, dozen, column and outside chips;
+  left click adds, right click removes.
+- Blackjack: dealer/player cards, hit/stand/double/forfeit.
+- Crash: curve, live multiplier, cash-out, commit/seed verification.
+- History: settled plays with net result.
+
+## Theme
+
+`style.css` mirrors the palette in `gui/src/theme.rs`: near-black ground,
+cyan focus, green for wins, gold for chips, felt green only for roulette.
+Do not introduce a second palette or hard-code colours in the render modules.
+
+## Security
+
+- The session cookie is HttpOnly and SameSite=Strict; JavaScript never reads
+  it and never stores a credential.
+- Notices are inserted with `textContent`, not HTML.
+- The app is play-money only. Do not add deposits, withdrawals, KYC or
+  real-money claims without a product decision and an ADR.