feat(wire): search_hit.v1 + citation.v1 require indexed_at + stale (fb-32)

Additive minor — schema_version unchanged. Existing v1 consumers
that ignore unknown fields stay compatible; consumers that validate
strictly will reject pre-fb-32 payloads, which matches the wire
contract escape hatch (recipient version >= producer required).

Cross-task placeholders: kebab-eval / kebab-tui synthetic test
fixtures pin UNIX_EPOCH + stale=false (same pattern as
hybrid.rs / vector.rs). These don't exercise staleness — Task 11
adds dedicated TUI staleness rendering tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/wire-schema/v1/citation.schema.json

@@ -4,7 +4,7 @@
   "title": "Citation v1",
   "description": "Stub schema — declares the schema_version label and the always-present fields. Variant-discriminated property validation lands in a later phase.",
   "type": "object",
-  "required": ["schema_version", "kind", "path", "uri"],
+  "required": ["schema_version", "kind", "path", "uri", "indexed_at", "stale"],
   "properties": {
     "schema_version": { "const": "citation.v1" },
     "kind": { "enum": ["line", "page", "region", "caption", "time"] },
@@ -14,6 +14,8 @@
     "page":    { "type": "object" },
     "region":  { "type": "object" },
     "caption": { "type": "object" },
-    "time":    { "type": "object" }
+    "time":    { "type": "object" },
+    "indexed_at": { "type": "string", "format": "date-time" },
+    "stale":      { "type": "boolean" }
   }
 }

docs/wire-schema/v1/search_hit.schema.json

@@ -16,7 +16,9 @@
     "citation",
     "retrieval",
     "index_version",
-    "chunker_version"
+    "chunker_version",
+    "indexed_at",
+    "stale"
   ],
   "properties": {
     "schema_version": { "const": "search_hit.v1" },
@@ -33,6 +35,8 @@
     "retrieval":       { "type": "object" },
     "index_version":   { "type": "string" },
     "embedding_model": { "type": ["string", "null"] },
-    "chunker_version": { "type": "string" }
+    "chunker_version": { "type": "string" },
+    "indexed_at":      { "type": "string", "format": "date-time" },
+    "stale":           { "type": "boolean" }
   }
 }