feat(principles): canonical_statement synthesis service + throttled backfill (Phase E groundwork, #152)

Grounded (INV-AH) multi-instance synthesis with drift guard + chair gate
(pending_review, G10). Single path used by backfill, MCP tool, nightly drain.
HELD from production run pending the principles-redesign (rename+cull, #152).

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

mcp-server/src/legal_mcp/server.py

@@ -465,6 +465,13 @@ async def canonical_halacha_get(canonical_id: str) -> str:
     return await plib.canonical_halacha_get(canonical_id)
 
 
+@mcp.tool()
+async def canonical_synthesize_pending(limit: int = 20) -> str:
+    """סנתז ניסוח-קנוני לעקרונות הממתינים (pending_synthesis) → pending_review (שער-יו"ר). V41 Phase 4.
+    מעוגן בציטוטי-המופעים (INV-AH) עם שער-drift. on-demand/burst; המסה הראשונית ב-backfill."""
+    return await plib.canonical_synthesize_pending(limit)
+
+
 # Documents
 @mcp.tool()
 async def document_upload(