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>

.github/workflows/merge-gates.yml

@@ -50,6 +50,14 @@ jobs:
       - name: Unit tests (incl. negative controls)
         run: cargo test --release --workspace
 
+      # plan2.md Phase H: the protocol + socket + replay/visibility gates are
+      # merge-blocking. These run the full HTTP/WebSocket/protocol/runtime path
+      # headlessly over real sockets (1k matches, 10k fuzz, 100 e2e, leak +
+      # resilience). The rendered-browser layer is NOT gated here — see
+      # web-gates.yml (advisory, blocked on CI infrastructure).
+      - name: Web protocol + socket gates (merge-blocking)
+        run: cargo test --release -p protocol -p game_runtime -p server -p web_assets -p web_client -p web_tests
+
       - name: Full merge-blocking acceptance gates
         env:
           MAGICKA_PROFILE: merge