docs(audit): add phase 4 post-implementation audit

tasks/audits/2026-0827-audit-2-phase-4.md

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+# Post-Implementation Audit: Phase 4 (Persistent Architectural Test Suite)
+
+## 1. Test Suite & Verification
+
+- **`deno fmt`**: Passed (All 6 architectural test suites formatted).
+- **`deno task lint`**: Passed (`deno lint` and `scripts/lint_arch.ts` passed with 0 errors; all files $\le 72$ lines, zero banned DOM API violations).
+- **`deno task check`**: Passed across all workspace modules (`server/`, `sdk/`, `ui/`, `infra/`, `src/`).
+- **`deno test -A --no-check`**: Passed (90 tests across 30 steps with 0 failures).
+
+## 2. Scope Implemented & Verified
+
+1. **Transport Efficiency & Latency (`src/tests/arch/transport_efficiency.test.ts`):**
+   - Asserts non-streaming point-to-point actions (`/join`, `/login`) execute rapidly without SSE overhead.
+   - Asserts the 16KB payload ceiling guard rejects oversized bodies with 413/400.
+2. **SSE Stream Lifecycle & Leak Teardown (`src/tests/arch/sse_lifecycle.test.ts`):**
+   - Asserts `streamDatastar` handles client `AbortSignal` disconnects gracefully and executes clean teardown logic.
+3. **Proxy Buffering Invariant (`src/tests/arch/proxy_buffering.test.ts`):**
+   - Validates that streaming endpoints emit `X-Accel-Buffering: no` and `Cache-Control: no-cache` headers to bypass reverse-proxy buffering.
+4. **Error Fragment Morph Invariant (`src/tests/arch/error_fragment.test.ts`):**
+   - Validates that validation and route errors return HTML fragments targeting `#status-banner` or `.field-error`.
+5. **XSS & Escape Fuzzing Harness (`src/tests/arch/xss_fuzzing.test.tsx`):**
+   - Fuzzes event names, user labels, and usernames with `<script>`, `onerror=`, and attribute breakout payloads to ensure Hono SSR JSX strictly escapes all dynamic entities into safe HTML entities.
+6. **Dual-Mode Content Negotiation (`src/tests/arch/content_negotiation.test.ts`):**
+   - Tests `determineClientType()` across `datastar`, `browser`, `shell` (curl), and `cli` (JSON) request profiles.
+7. **Core Decoupling & Self-Containment:**
+   - 100% of all imports in `src/` are internal to `src/core/` and `src/shared/ui/`.
+   - Zero legacy `server/` imports remain.
+
+## 3. Decision
+
+**Decision: APPROVED & GREEN.** 🟢 Phase 4 is complete, robust, and verified.