feat(2): Q4.1 partial — matrix-synapse Phase-2 code (NOT YET cold-verified end-to-end)

Code-only commit. The Phase-2 functional tests + PARITY.md are written and locally consistent,
but the e2e cold-verify on cc-ci is BLOCKED by abra deploy timing out (900s) on the
matrix-synapse stack. The deploy hits the orchestrator's wait_healthy timeout — synapse +
postgres-autoupgrade are too slow on this host (28GB disk, 3.5GB RAM, single node).

Even after pruning Docker images (freed disk from 90% → 55% used), the deploy still times out.
Root cause appears to be CPU/IO-bound startup on this host rather than disk space.

What's landed (code-only):
- tests/matrix-synapse/PARITY.md: parity table; the 3 recipe-maintainer shell-script tests
  (compress_state / test_complexity_limit / test_purge) deferred with technical rationale
  (operational regressions against persistent state — incompatible with the ephemeral per-run
  model). Phase-2 health_check added (the corpus has no health_check.py).
- tests/matrix-synapse/functional/test_health_check.py: GET /_matrix/client/versions → 200 + JSON.
- tests/matrix-synapse/functional/test_federation_version.py: GET /_matrix/federation/v1/version
  → 200, asserts server.name='Synapse' + non-empty server.version (plan §4.3 prescribed).
- tests/matrix-synapse/functional/test_register_and_message.py: plan §4.3 prescribed test —
  registers two users via the public client API (m.login.dummy UIAA flow), logs in, creates a
  private_chat room, invites + joins user_b, sends an m.room.message with a uuid marker, reads
  the room's messages, asserts the marker appears in user_b's view. Non-vacuous full client-API
  roundtrip.
- tests/matrix-synapse/recipe_meta.py: EXTRA_ENV adds ENABLE_REGISTRATION=true (lets the test
  use public client registration; admin endpoints aren't routed publicly by this recipe) and
  TIMEOUT=900 (overrides the recipe's default 300s abra-deploy convergence timeout).

**Cold-verify status: BLOCKED on cc-ci host capacity for matrix-synapse deploys** — needs
operator review (more disk / RAM / a heavier-recipe sequencing strategy). Filed in JOURNAL-2 +
PushNotification.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/matrix-synapse/functional/test_health_check.py

@@ -0,0 +1,29 @@
+"""matrix-synapse — health-check parity port (Phase 2 P2).
+
+SOURCE: there is no `recipe-info/matrix-synapse/tests/health_check.py` in the recipe-maintainer
+corpus (the corpus's matrix-synapse tests are shell scripts targeting a persistent instance —
+see DECISIONS.md for the Q4 deeper-port deferral). This file is a Phase-2 health_check ALIGNED
+with the parity-port convention: HTTP 200 from the client API versions endpoint, which is the
+recipe's HEALTH_PATH and the canonical "is synapse up" signal.
+
+Runs in the custom tier against the shared post-install live deployment.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import sys
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "..", "runner"))
+from harness import http as harness_http  # noqa: E402
+
+
+def test_synapse_client_versions_returns_json(live_app):
+    """Verify /_matrix/client/versions returns 200 + a JSON document with a non-empty versions list."""
+    url = f"https://{live_app}/_matrix/client/versions"
+    status, body = harness_http.retry_http_get(url, expect_status=200, max_wait=60, interval=3)
+    assert status == 200, f"GET {url} HTTP {status} (expected 200)"
+    assert isinstance(body, dict) and isinstance(body.get("versions"), list) and body["versions"], (
+        f"GET {url} did not return Matrix client-versions document: {body!r}"
+    )