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/recipe_meta.py

@@ -3,3 +3,14 @@
 # (DG5) is detected quickly instead of waiting the default 300s HTTP timeout.
 DEPLOY_TIMEOUT = 120
 HTTP_TIMEOUT = 90
+
+# Rungs this recipe INTENTIONALLY skips, each with a reason. Any essential rung skipped (N/A) and NOT
+# listed here is reported as an *unintentional* skip (a coverage gap to fill or declare). A skip still
+# caps the level either way — the harness never claims a rung it did not verify; this only records
+# that the skip is deliberate. (The level ladder is the four essential rungs install/upgrade/
+# backup_restore/functional; integration + recipe-local are optional and not leveled.)
+# custom-html-tiny is a stateless static-web-server, so it has no backup surface:
+EXPECTED_NA = {
+    "backup_restore": "stateless static file server: serves an ephemeral content volume seeded at "
+    "deploy, with no persistent/user data to back up or restore (no backupbot.backup label)",
+}