feat(digests): case-contextual digest radar — surface unlinked digests as chair leads (X12)

A digest pointing at a ruling we don't hold yet ("unlinked") was captured globally
(missing_precedents inbox) but never surfaced IN THE CONTEXT of the case being
decided — so a relevant ruling known only via a digest could fall through the cracks
at the moment it matters. Adds the case-contextual radar:

- db.search_digests_semantic: new `linked_only` filter (False = unlinked-only target set).
- digest_library.case_digest_radar(case_number): builds the case topic from title +
  appeal_subtype + the analyst's claims, embeds once, matches against UNLINKED digests,
  and returns leads enriched with the underlying ruling's gap status + suggested action
  (new_lead / gap_open / fetched / available_link).
- MCP tool `digest_radar` + endpoint GET /api/cases/{n}/digest-radar (for the agent and
  the future case-page lead).

INV-DIG1 preserved: radar only — every lead points at the underlying RULING (fetch /
upload / link), never cites the digest. Read-only.

Validated on 8124-09-24 (היטל השבחה): 5 on-topic unlinked digests, scores 0.64–0.72.
The visible case-page panel is a separate UI change → goes through the design gate.

Invariants: G2 (reuses the one digest semantic-search + gap/citation resolvers),
INV-DIG1 (no digest citation), INV-DIG3 (gap surfacing). No schema change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/db.py

@@ -5018,14 +5018,24 @@ async def search_digests_semantic(
     subject_tag: str = "",
     concept_tag: str = "",
     limit: int = 10,
+    linked_only: bool | None = None,
 ) -> list[dict]:
     """Pure-semantic search over the digests radar (X12). Single vector per row
     (no chunks/halachot), so no RRF here — see X12 §6. Joins the linked ruling's
-    citation when present so the researcher sees the pointer target directly."""
+    citation when present so the researcher sees the pointer target directly.
+
+    ``linked_only``: None = all digests (default); False = only UNLINKED digests
+    (``linked_case_law_id IS NULL`` — rulings we don't hold yet, the case radar's
+    target set); True = only linked digests.
+    """
     pool = await get_pool()
     conditions = ["d.embedding IS NOT NULL"]
     params: list = [query_embedding, limit]
     idx = 3
+    if linked_only is True:
+        conditions.append("d.linked_case_law_id IS NOT NULL")
+    elif linked_only is False:
+        conditions.append("d.linked_case_law_id IS NULL")
     if practice_area:
         conditions.append(f"d.practice_area = ${idx}")
         params.append(practice_area)