test(ci): the e2e job summary must name why a spec failed (refs #161)

#161 lost a 36-minute verify-stack job whose only surviving output was a
single ✘ line: the per-spec summary (#136) renders a verdict icon and nothing
else, so a red e2e still costs a log dive — and when the log is truncated or
the run is killed, there is nothing to dive into.

Adds a stdlib assert-based self-check for infra/playwright-summary.py (no
framework) and rides it on `make unit` so CI catches a broken summary.
Fails with "AssertionError: Test timeout of 90000ms exceeded" not in the
rendered markdown.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Makefile

@@ -71,8 +71,11 @@ build:
 
 ## unit: run unit tests (excludes the container-backed Integration lane)
 # TRX per test project (→ TestResults/) feeds the CI per-service summary (#136); harmless locally.
+# The CI reporting scripts are stdlib Python with their own assert-based self-checks (#161) — they
+# ride this lane so a broken job summary is caught by CI rather than by the next red pipeline.
 unit:
 	dotnet test $(SLN) -c Release --filter "Category!=Integration" --logger trx --results-directory TestResults
+	python3 infra/test_playwright_summary.py
 
 ## mutation: run the Stryker.NET ratchet on each service with branching logic (fails below baseline)
 # Stryker is pinned as a local dotnet tool (.config/dotnet-tools.json); `tool restore`