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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+# 0003 — Browser frontend through a server-side gateway
+
+Status: accepted, 2026-11-08
+
+## Context
+
+The game already runs over TLS/TCP with a newline-delimited text protocol. A
+browser cannot open a TCP socket, and putting game rules into JavaScript would
+create a second implementation of payout, session and forfeit logic — exactly
+what the project forbids.
+
+The site must stay a website in the ordinary sense: open `zagan.bet`, log in,
+play. Nothing to install. Balances are play credits; there are no deposits,
+withdrawals or KYC in this decision.
+
+## Decision
+
+1. **The browser is a third frontend over `ClientCore`.** It sends `Command`
+   JSON and renders `View` JSON. It contains no rules, no wire-protocol parser
+   and no balance arithmetic.
+2. **A Rust gateway, `webd/`, owns each session.** One blocking thread per
+   browser session, one `ClientCore`, and one ordinary client TLS connection
+   upstream. The existing `client::transport` and `ClientCore` are reused. No
+   async runtime and no new concurrency style.
+3. **`webd/` is a separate workspace member.** Like `gui/`, its dependencies
+   (tungstenite, HTTP cookie handling) must never enter the TUI, GUI or server
+   build graph.
+4. **Everything is same-origin and operator-configured.** nginx terminates
+   TLS, serves the static app at `/play/`, and proxies `/ws` to the gateway on
+   loopback. The browser never sees a certificate, there is no CORS, and the
+   upstream address (`ZAGAN_UPSTREAM`, default `127.0.0.1:11525`) and SPKI
+   fingerprint (`ZAGAN_UPSTREAM_FINGERPRINT`, required) are gateway config.
+   A mismatch refuses to connect and logs both fingerprints. There is no TOFU
+   and no unsafe mode for the upstream leg.
+5. **Session-only authentication.** Log in per visit. The gateway issues a
+   random HttpOnly, SameSite=Strict, Secure session cookie; the cookie covers a
+   refresh and the grace window only. No credential is stored in the browser or
+   in the gateway. A new connection with the same cookie takes over the session
+   and the old socket is told it was replaced.
+6. **60-second grace window.** When the last browser socket for a session
+   closes, the session thread keeps the upstream connection and `ClientCore`
+   alive for 60 seconds (configurable). A reconnecting socket with the same
+   cookie reattaches and immediately receives the current `View`. Expiry closes
+   upstream, and the server forfeits a live hand per the existing disconnect
+   rule. There is no wire-level resume: the gateway, not the browser, holds the
+   TCP connection.
+7. **Vanilla TypeScript + esbuild.** DOM for login, lobby, history and notices;
+   one canvas for the game surface. No framework runtime. Theme tokens are
+   reused from the desktop palette; felt green is reserved for roulette.
+8. **Web v1 has full parity after trust.** Register, log in, lobby, all four
+   games, history, log out, notices, refresh grace. The connect/trust screens
+   do not exist in the browser because the gateway owns the upstream key.
+
+## Consequences
+
+- One implementation of the rules and parsers remains authoritative, and the
+  TUI/GUI/gateway all exercise the same `ClientCore` tests.
+- The gateway becomes an operational component: it needs a service unit,
+  deployment, logging, a required upstream fingerprint, and a restart story.
+  A gateway restart loses live sessions and therefore forfeits their rounds;
+  this is accepted for v1.
+- The browser cannot recover from a gateway crash by itself. The player logs in
+  again.
+- A 60-second grace window means a live round can outlive a refresh, but not a
+  closed laptop for a minute and a half.
+- WebSocket frames are same-origin JSON, so the page cannot be used as an open
+  proxy by another origin. The gateway also checks the `Origin` header against
+  `ZAGAN_WEB_ORIGIN`.
+- The web app is play-money only. Adding payments later would be a separate
+  product and compliance decision, not a UI change.
+
+## Alternatives considered
+
+- **WASM `ClientCore` in the browser, dumb relay**: reuses Rust in the page,
+  but needs transport, TOFU and credential shims in wasm and a multi-megabyte
+  download for every player. Rejected.
+- **JavaScript rewrite speaking the game protocol**: fastest UI iteration, but
+  a second implementation of rules and payout invariants. Rejected.
+- **Tokio/axum gateway**: idiomatic WebSocket server, but a large async tree
+  and a second concurrency style for one blocking, low-traffic product.
+  Rejected.
+- **Long-lived remember-me in the gateway**: better return visits, but a new
+  credential store and revocation flow. Deferred; session-only first.
+- **Protocol-level resume tokens**: survives a gateway restart, but is a wire
+  change and more state on the server. Deferred.