feat(8.0): u64 BigInt graph provider contract — punch list a-d,g,h

GraphIndexProvider now speaks BigInt at the boundary (D.2 mirror):
- getNeighbors/getVerbIdsBySource/getVerbIdsByTarget take entity ints and
  return entity/verb ints as bigint[]
- new REQUIRED verbIntsToIds(bigint[]) batch reverse resolver (L.7
  identity-fingerprint design — verb ids are UUIDs by contract, so the
  provider-side interning is losslessly reversible)
- addVerb(verb, sourceInt, targetInt) returns the interned verb int;
  removeVerb(verbId) joins the contract

Coordinator (brainy.ts) owns ALL UUID <-> int conversion: getOrAssign on
writes, getInt on reads (unmapped UUID -> empty result without calling the
provider), getUuid / verbIntsToIds on returns, plus a bounded ~100k-entry
insertion-order warm cache for verb-int -> verb-id pairs fed by addVerb
returns and resolver results. GraphVerb gains derived sourceInt/targetInt
(populated at add time, never persisted). findConnectedSubtype gains a
native fast path that routes single-type single-subtype outgoing BFS
through the provider when available.

JS GraphAdjacencyIndex satisfies the contract while staying string/u32-keyed
internally: entity ints resolve through the shared entity-id mapper (threaded
in by the coordinator on init/fork/checkout), verb ints come from an
in-process append-only interning map re-derived from storage on
rebuild/cold-start. ColumnStoreProvider widens the same way: addEntity/
removeEntity take bigint, sortTopK/filteredSortTopK return bigint[].

relate() now rejects a caller-supplied id with a teaching error — verb ids
are brainy-generated UUIDs by contract in 8.0 (previously a passed id was
silently ignored). No Roaring64 provider-boundary decode site exists yet;
the JS-internal column store stays Roaring32 and the Treemap decoder lands
with the first consumer of provider-returned filter buffers.

Public brain API unchanged. 1413 tests green (+10 new BigInt contract tests).

tests/unit/brainy/relate-duplicate-optimization.test.ts

@@ -151,8 +151,12 @@ describe('Duplicate Check Optimization', () => {
       type: VerbType.RelatesTo
     })
 
-    // Access GraphIndex to ensure verb is cached
-    const verbIds = await (brain as any).graphIndex.getVerbIdsBySource(entityA)
+    // Access GraphIndex to ensure verb is cached (8.0 BigInt boundary:
+    // UUID → entity int in, verb ints out, resolved back via verbIntsToIds)
+    const idMapper = (brain as any).metadataIndex.getIdMapper()
+    const sourceInt = BigInt(idMapper.getInt(entityA)!)
+    const verbInts: bigint[] = await (brain as any).graphIndex.getVerbIdsBySource(sourceInt)
+    const verbIds = await (brain as any).graphIndex.verbIntsToIds(verbInts)
     expect(verbIds).toContain(relationId1)
 
     // Attempt duplicate (should use cached verb)