feat(harness): P1 — single registry-backed meta loader (rcust)

One loader: runner/harness/meta.py::load(recipe) -> RecipeMeta (frozen dataclass,
attribute access), backed by the declarative KEYS registry (14 final keys + 3
P2-deprecated). The ONLY exec() of tests/<recipe>/recipe_meta.py. Validation per
the locked decision: unknown ALL-CAPS top-level name or type mismatch = MetaError
(hard error at load); underscore-prefixed names recipe-private; callables only on
hook-typed keys.

Migrated all six legacy loaders (spec §4 L1–L6):
- run_recipe_ci.py::_load_meta deleted; orchestrator loads once, passes meta down
- tests/conftest.py::_recipe_meta deleted; meta fixture returns full RecipeMeta (R3)
- lifecycle.py::_recipe_extra_env/_recipe_meta_flag deleted; deploy_app takes meta
- deps.py::declared_deps deleted; callers read meta.DEPS
- canonical.py::is_enrolled reads through meta.load()
- screenshot.py now actually receives SCREENSHOT through the orchestrator path (R2
  fix; proven by unit test through the real load path)

Mumble private constants underscore-prefixed (_WELCOME_TEXT_MARKER/_MAX_USERS) +
importers fixed. New tests/unit/test_meta.py (all-recipes-load-clean typo gate,
MetaError cases, spec §2 baseline defaults, underscore exemption, doc sync). Docs
§4 key table now GENERATED from the registry (scripts/gen-meta-docs.py); drift
fails CI.

Verified on cc-ci: cc-ci-run -m pytest tests/unit -q -> 175 passed; scripts/lint.sh -> PASS.

runner/harness/screenshot.py

@@ -33,12 +33,19 @@ def screenshot_path(run_artifact_dir: str) -> str:
     return os.path.join(run_artifact_dir, "screenshot.png")
 
 
-def _load_screenshot_hook(recipe_meta: dict | None):
+def _load_screenshot_hook(recipe_meta):
     """Return the recipe's optional SCREENSHOT hook (a callable) if it declared one, else None.
-    The hook drives Playwright to a safe post-login view; default is the landing page."""
-    if not recipe_meta:
+    The hook drives Playwright to a safe post-login view; default is the landing page.
+
+    `recipe_meta` is the loaded RecipeMeta (rcust P1 — the single loader actually delivers
+    SCREENSHOT now; under the old L1 allowlist the key never arrived, spec §8 R2). A plain dict
+    is still accepted for direct/manual callers."""
+    if recipe_meta is None:
         return None
-    hook = recipe_meta.get("SCREENSHOT")
+    if isinstance(recipe_meta, dict):
+        hook = recipe_meta.get("SCREENSHOT")
+    else:
+        hook = getattr(recipe_meta, "SCREENSHOT", None)
     return hook if callable(hook) else None