feat(2): Q0.3/Q1.1 — custom-html PARITY + functional + playwright (Phase 2)

- tests/custom-html/PARITY.md: parity mapping (health_check.py ported);
  recipe-specific tests recorded with rationale; backup data-integrity +
  playwright sections.
- tests/custom-html/functional/test_health_check.py: parity port of
  recipe-info/custom-html/tests/health_check.py — SOURCE comment included.
- tests/custom-html/functional/test_content_roundtrip.py: NEW recipe-specific —
  write a marker into the served volume, fetch over HTTPS, assert exact bytes.
- tests/custom-html/functional/test_content_type_header.py: NEW recipe-specific —
  prove nginx returns text/html for .html and text/plain for .txt (MIME mapping).
- tests/custom-html/playwright/test_browser_smoke.py: P6 browser smoke (renders
  HTML, no console errors). Standalone Phase-2 custom-stage version.

Verified cold on cc-ci (STAGES=install,custom): 5 assertions all PASS in one
run (install generic + install overlay + content roundtrip + content type +
health check + browser smoke), deploy-count=1.

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

tests/custom-html/functional/test_content_roundtrip.py

@@ -0,0 +1,54 @@
+"""custom-html — recipe-specific functional test (Phase 2 P3, ≥2 beyond parity).
+
+The recipe IS a content server (nginx serving /usr/share/nginx/html). Its characteristic behavior is
+**serve persisted content**, not "returns 200". So write a uniquely-marked file into the served
+volume and fetch it back over HTTPS — proves the app serves what's in the volume, not a hardcoded
+default page.
+
+Runs in the custom tier against the shared post-install deployment (live_app). No deploy/teardown.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+import uuid
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "..", "runner"))
+from harness import http as harness_http, lifecycle  # noqa: E402
+
+
+def test_content_roundtrip(live_app):
+    """Write a uniquely-marked .txt file into nginx's served volume, fetch it over HTTPS, assert the
+    exact bytes round-trip. Non-vacuous: a stale page or misrouted backend would not return our
+    randomly-generated content."""
+    marker = f"ccci-roundtrip-{uuid.uuid4().hex}"
+    # written into the served volume; nginx routes /<filename> to /usr/share/nginx/html/<filename>
+    filename = f"ccci-roundtrip-{uuid.uuid4().hex[:12]}.txt"
+    lifecycle.exec_in_app(
+        live_app,
+        ["sh", "-c", f"printf %s {marker} > /usr/share/nginx/html/{filename}"],
+    )
+
+    url = f"https://{live_app}/{filename}"
+    # short retry: nginx serves the file off the live volume as soon as it lands; the retry is
+    # belt-and-suspenders against any FS-cache latency.
+    deadline_wait = 15
+    last_status, last_body = 0, ""
+    for _ in range(deadline_wait):
+        last_status, last_body = lifecycle.http_fetch(live_app, f"/{filename}", timeout=10)
+        if last_status == 200 and last_body == marker:
+            break
+        import time
+
+        time.sleep(1)
+    assert last_status == 200, f"GET {url} -> HTTP {last_status} (expected 200)"
+    assert last_body == marker, (
+        f"GET {url} returned {last_body!r}, expected {marker!r} — "
+        "the served volume did not round-trip"
+    )
+    # Also prove it via the harness_http helper (consistent with Phase-2 canonical API)
+    status, parsed = harness_http.http_get(url)
+    assert status == 200
+    # plain text, not JSON — parsed should be None (http_get only returns json_or_None)
+    assert parsed is None or parsed == marker