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>

crates/kebab-tui/tests/ask.rs

@@ -42,6 +42,10 @@ fn make_answer(grounded: bool, refusal: Option<RefusalReason>, body: &str) -> An
                 end: 14,
                 section: Some("Section A".into()),
             },
+            // fb-32: TUI ask test fixture pinned to UNIX_EPOCH + stale=false;
+            // staleness rendering covered in dedicated tests (Task 11).
+            indexed_at: OffsetDateTime::UNIX_EPOCH,
+            stale: false,
         }],
         grounded,
         refusal_reason: refusal,