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_tests/e2e/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "magicka-web-e2e",
+  "version": "0.1.0",
+  "private": true,
+  "description": "Playwright rendered-browser E2E for the Magicka VM web game.",
+  "scripts": {
+    "test": "playwright test"
+  },
+  "devDependencies": {
+    "@playwright/test": "^1.40.0"
+  }
+}

crates/web_tests/e2e/playwright.config.js

@@ -0,0 +1,28 @@
+// Playwright config for the rendered-browser E2E layer. This complements the
+// headless Rust protocol E2E in `crates/web_tests/tests/e2e.rs`: here a real
+// Chromium drives the actual DOM client. It boots the real server (short turns)
+// via `webServer` so `npm test` is self-contained.
+//
+// Requires Node + `npx playwright install chromium`. The Rust CI gates do not
+// depend on this; it is the optional rendered-browser proof.
+
+const { defineConfig } = require("@playwright/test");
+
+module.exports = defineConfig({
+  testDir: "./specs",
+  timeout: 30000,
+  expect: { timeout: 10000 },
+  use: {
+    baseURL: "http://127.0.0.1:8099",
+    headless: true,
+  },
+  webServer: {
+    // Build once, then run the server with fast turns for E2E.
+    command: "cargo run --release -p server --bin magicka-server",
+    cwd: "../../..",
+    env: { MAGICKA_ADDR: "127.0.0.1:8099", MAGICKA_TURN_MS: "1500" },
+    url: "http://127.0.0.1:8099/",
+    reuseExistingServer: true,
+    timeout: 120000,
+  },
+});

crates/web_tests/e2e/specs/play.spec.js

@@ -0,0 +1,43 @@
+// Rendered-browser E2E: a real Chromium joins a match, the turn timer runs, the
+// player edits + saves a rune program, casts, sees filtered results, and replays
+// the match — verifying the recorded replay hashes match what was seen live.
+//
+// This is the browser layer of plan2.md Phase H ("Playwright end-to-end tests").
+
+const { test, expect } = require("@playwright/test");
+
+test("a player can join, cast, and replay a browser match", async ({ page }) => {
+  await page.goto("/");
+
+  // Connects to the authoritative server.
+  await expect(page.locator("#conn")).toHaveText("connected", { timeout: 10000 });
+
+  // The turn timer is running (header shows a turn + countdown).
+  await expect(page.locator("#timer")).toContainText("turn", { timeout: 10000 });
+
+  // The arena rendered with the player's marker.
+  await expect(page.locator(".cell.self")).toHaveCount(1, { timeout: 10000 });
+
+  // Domains panel shows the hidden-state redaction notice (visibility layer).
+  await expect(page.locator("#redactions")).toContainText("withheld", { timeout: 10000 });
+
+  // Edit a rune program: add a couple of runes and save.
+  await page.fill("#tok-a", "1");
+  await page.fill("#tok-b", "2");
+  await page.click("#btn-add");
+  await page.click("#btn-add");
+  await page.click("#btn-save");
+  // Observed diagnostics appear (names/counts only).
+  await expect(page.locator("#diag-body")).toContainText("known reads", { timeout: 10000 });
+
+  // Cast and wait for a resolved-turn log line.
+  await page.click("#btn-cast");
+  await expect(page.locator("#log")).toContainText("cast a rune program", { timeout: 15000 });
+
+  // Request and step the replay; the client verifies hashes vs. what it saw live.
+  await page.click("#btn-replay");
+  await expect(page.locator("#replay-status")).toContainText("replay loaded", { timeout: 10000 });
+  await page.click("#btn-replay-step");
+  // A matching hash line is shown (no MISMATCH).
+  await expect(page.locator(".hash-bad")).toHaveCount(0);
+});