feat(harness): intentional skips + custom-html-tiny functional test; 4-rung ladder (#6)

Declare intentional skips + custom-html-tiny functional test; 4-rung level ladder

- recipe_meta.EXPECTED_NA = {rung: reason} lists intentionally-skipped rungs; any
  essential rung skipped and not listed is unintentional. Skips still cap the level
  (never inflate). results.json: skips:{intentional,unintentional} + level_cap_rung.
- Level ladder = the four essential rungs (install, upgrade, backup/restore,
  functional; top = L4). integration & recipe-local are optional, not leveled
  (SSO still enforced for the run verdict, unchanged).
- Card shows skipped rungs as INTENTIONAL SKIP (green, reason below) / UNINTENTIONAL
  SKIP (amber); level badge gains an expected/gap? third segment.
- custom-html-tiny: functional serve test (exact-byte round-trip + 404); declares
  backup_restore intentionally skipped (stateless static server).

Independently verified by the adversary: 138 unit tests pass cold; live full-stage
run on custom-html-tiny green (upgrade tier ran; level 2; correct skips/badge);
clean teardown.

tests/custom-html-tiny/functional/test_serves_content.py

@@ -0,0 +1,87 @@
+"""custom-html-tiny — recipe-specific functional test (static-web-server).
+
+Proves the deployed static-web-server is *actually serving files from its `content` volume* with real
+file-server semantics, not merely returning 200 from a Traefik fallback or a generic stub:
+
+  1. exact-byte round-trip — write a uniquely-named file with random content into the served volume,
+     fetch it over HTTPS, and assert the bytes come back verbatim. Non-vacuous: the content is random
+     per run, so only a server that reads this file off the volume can pass.
+  2. real 404 — a random non-existent path returns 404, proving directory/file semantics (a
+     200-everything stub or mis-routed host would not 404).
+
+The recipe's image (joseluisq/static-web-server) is shell-less (scratch-based) and its content volume
+is seeded via the install_steps.sh host-mountpoint mechanism — so this test writes its probe file the
+same way (resolve the swarm volume's mountpoint with `docker volume inspect`, write directly) rather
+than `docker exec`-ing in a container that has no shell.
+
+Runs in the custom tier against the shared post-install deployment (the `live_app` fixture is its
+per-run domain). Mirrors install_steps.sh: the app's content volume is named `<stack>_content`, where
+`stack` is the domain with dots replaced by underscores; HTTP_SUBDIR is empty, so the volume root is
+served at `/`.
+"""
+
+from __future__ import annotations
+
+import contextlib
+import os
+import ssl
+import subprocess
+import urllib.error
+import urllib.request
+import uuid
+
+
+def _served_dir(domain: str) -> str:
+    """Host mountpoint of the app's served `content` volume (same naming as install_steps.sh)."""
+    vol = f"{domain.replace('.', '_')}_content"
+    out = subprocess.run(
+        ["docker", "volume", "inspect", vol, "--format", "{{.Mountpoint}}"],
+        capture_output=True,
+        text=True,
+        check=True,
+    )
+    mountpoint = out.stdout.strip()
+    assert mountpoint, f"could not resolve mountpoint for volume {vol!r}"
+    return mountpoint
+
+
+def _get(url: str) -> tuple[int, bytes]:
+    """GET the URL; return (status, body). A 4xx/5xx is returned, not raised (we assert on the code).
+    TLS verification is relaxed: the served wildcard cert is validated separately by the infra check;
+    here we care only about the app's response."""
+    ctx = ssl.create_default_context()
+    ctx.check_hostname = False
+    ctx.verify_mode = ssl.CERT_NONE
+    try:
+        with urllib.request.urlopen(url, timeout=20, context=ctx) as resp:
+            return resp.status, resp.read()
+    except urllib.error.HTTPError as e:
+        return e.code, e.read()
+
+
+def test_static_file_roundtrip_and_404(live_app):
+    """Write a random file into the served volume → fetch it → bytes match; and a missing path 404s."""
+    served = _served_dir(live_app)
+    token = uuid.uuid4().hex
+    name = f"ccci-probe-{token}.txt"
+    body = f"cc-ci-functional-{token}\n".encode()
+    path = os.path.join(served, name)
+    with open(path, "wb") as fh:
+        fh.write(body)
+    try:
+        status, got = _get(f"https://{live_app}/{name}")
+        assert status == 200, f"served probe file returned {status} (expected 200)"
+        assert got == body, (
+            f"content round-trip mismatch: served {got!r}, wrote {body!r} "
+            "(static-web-server not serving the content volume?)"
+        )
+
+        # A random non-existent path must 404 — proves real static-file semantics, distinguishing a
+        # working server from a 200-everything stub or a mis-routed Traefik fallback.
+        miss_status, _ = _get(f"https://{live_app}/ccci-missing-{uuid.uuid4().hex}.txt")
+        assert miss_status == 404, (
+            f"missing path returned {miss_status} (expected 404 — generic 200-returner / mis-route?)"
+        )
+    finally:
+        with contextlib.suppress(OSError):
+            os.remove(path)