Finish FLEX-WP-0019 layer-model v0.7 conformance

Close the remaining PDP obligations: mechanical layer declaration check,
registry-snapshot digest in provenance, explicit allow TTL, per-input-class
freshness deadlines, and the published decision-record contract. Document
the canonical request digest as the §6.4.2 replay test.

Assistant: grok
Assistant-Session: 01a06256-fb71-7102-b3a9-27e6734257d0

schemas/policy_package.schema.json

@@ -25,7 +25,12 @@
     },
     "caring": {"$ref": "#/$defs/caring_policy_metadata"},
     "activation": {"type": "object", "additionalProperties": true},
-    "metadata": {"type": "object", "additionalProperties": true}
+    "metadata": {"type": "object", "additionalProperties": true},
+    "allow_ttl": {
+      "type": "string",
+      "minLength": 1,
+      "description": "Go duration bounding every allow this package produces (e.g. 15m). Omit to use the engine default of 15m. The values none and 0s mean no stated end; the engine denies those allows."
+    }
   },
   "$defs": {
     "caring_policy_metadata": {