Add web game (plan2.md) on the independent runtime, merge-blocking gates

Builds the browser game around the existing Rust runtime: a window into the
universe, not a second simulation. Pure std, no external crates.

New crates:
- protocol: versioned, hashable client/server messages + hand-rolled JSON value
  and total parser (malformed packet -> Err, never panic).
- game_runtime: authoritative match state. Resolves turns through the
  INDEPENDENT interpreter (runtime_under_test::native_resolve), not the
  reference engine; filters visibility/knowledge; records and regenerates
  replays. A match is a pure function of (seed, roster, ordered inputs).
- web_assets/web_client: embedded browser client (arena, rune editor,
  knowledge panels, replay viewer) + static HTTP delivery.
- server: std::net HTTP + WebSocket (hand-rolled SHA-1/base64/RFC-6455 framing),
  turn timer, disconnect handling, panic-proof dispatch, poison-tolerant lock.
- web_tests: dependency-free WebSocket test client + Phase H gates.

Trust hardening per review:
- game_runtime no longer delegates to reference_runtime::execute; it runs the
  independent interpreter that the runtime-equivalence gate proves correct.
- Protocol/socket/replay/visibility/resilience gates are merge-blocking
  (added to the merge_group-required job in merge-gates.yml): 1k matches/0
  drift, 10k fuzz/0 panics, 100 headless socket E2E, 0 hidden-state leaks.
- Rendered-browser E2E is marked EXTERNAL-BLOCKED: Playwright runs advisory-only
  (continue-on-error, artifacts) until CI infrastructure with a browser exists;
  it is treated as unsatisfied, not green. The headless 100-match gate is
  labeled protocol-level coverage, not rendered-browser coverage.
- README documents the hand-rolled crypto/parser audit risk explicitly.

Fixes an integer-overflow panic in observed-volatility inference (i64 sum /
abs near i64::MIN) that could poison the server mutex.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/web_client/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "web_client"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[dependencies]
+web_assets = { path = "../web_assets" }

crates/web_client/src/lib.rs

@@ -0,0 +1,54 @@
+//! `web_client` — the client delivery layer. It owns *how* the embedded
+//! [`web_assets`] reach the browser (the HTTP response framing), keeping the
+//! raw asset bytes (`web_assets`) separate from delivery concerns. The server
+//! depends on this crate, not on `web_assets` directly.
+
+pub use web_assets::{resolve, Asset};
+
+/// Build a complete HTTP/1.1 response for a static GET path. Returns `None`
+/// for unknown paths so the caller can emit a 404.
+pub fn http_response(path: &str) -> Option<Vec<u8>> {
+    let asset = resolve(path)?;
+    let body = asset.body.as_bytes();
+    let mut out = Vec::with_capacity(body.len() + 128);
+    let header = format!(
+        "HTTP/1.1 200 OK\r\nContent-Type: {}\r\nContent-Length: {}\r\nCache-Control: no-cache\r\nConnection: close\r\n\r\n",
+        asset.content_type,
+        body.len()
+    );
+    out.extend_from_slice(header.as_bytes());
+    out.extend_from_slice(body);
+    Some(out)
+}
+
+/// The canonical 404 response.
+pub fn not_found() -> Vec<u8> {
+    let body = b"404 not found";
+    let mut out = Vec::new();
+    let header = format!(
+        "HTTP/1.1 404 Not Found\r\nContent-Type: text/plain\r\nContent-Length: {}\r\nConnection: close\r\n\r\n",
+        body.len()
+    );
+    out.extend_from_slice(header.as_bytes());
+    out.extend_from_slice(body);
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn serves_shell() {
+        let r = http_response("/").unwrap();
+        let s = String::from_utf8_lossy(&r);
+        assert!(s.starts_with("HTTP/1.1 200 OK"));
+        assert!(s.contains("text/html"));
+        assert!(s.contains("Magicka VM"));
+    }
+
+    #[test]
+    fn unknown_path_is_none() {
+        assert!(http_response("/secret").is_none());
+    }
+}