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).

src/utils/paramValidation.ts

@@ -524,6 +524,21 @@ export function validateUpdateParams(params: UpdateParams): void {
  * Validate relate parameters
  */
 export function validateRelateParams(params: RelateParams): void {
+  // 8.0 verb-id contract (L.7): verb ids are UUIDs, generated by brainy.
+  // RelateParams has no `id` field — an untyped caller passing one would
+  // previously have it silently ignored (a generated UUID was used instead).
+  // Teach instead of surprise: explain the contract and how ids actually work.
+  const suppliedId = (params as RelateParams & { id?: unknown }).id
+  if (suppliedId !== undefined) {
+    throw new Error(
+      `relate() does not accept a custom id (got: ${JSON.stringify(suppliedId)}). ` +
+      'Verb ids are UUIDs by contract in 8.0 — brainy generates one for every ' +
+      'relationship and returns it from relate(). Graph-index providers key ' +
+      'verb-int interning on the raw UUID bytes, so custom verb ids are not ' +
+      'supported. Remove the id field and use the returned id instead.'
+    )
+  }
+
   // Universal truths
   if (!params.from) {
     throw new Error('from entity ID is required')