fix(update-seam): the metadata crossing never carries BigInt endpoint ints

resolveVerbEndpointInts mirrors the resolved u64 endpoint ints onto the
verb object itself as BigInt (verb.sourceInt/targetInt) for the graph legs'
own params. The live verb path's delete legs then reused that same object
as the metadata-index crossing — and the seam's metadata is JSON-safe by
contract (a native provider serializes it; u64 as Number corrupts above
2^53), so JSON.stringify threw and the whole transaction aborted. Found by
the first joint pair gate; four downstream suites red from one crossing.

The crossing now routes through a JSON-safe view that drops BigInt-valued
top-level keys — endpoint ints ride their own op params on the graph legs,
exactly as designed, and never the metadata crossing. Applied at the
retraction helper (cascade + unrelate + transact mirrors) and
updateRelation's remove leg.

Pinned by driving the exact shape (relate resolves ints, remove cascades
the same object) through a provider shim enforcing the JSON contract —
red-proved against the unfixed path (the joint gate's verbatim error),
green with the fix.

src/brainy.ts

@@ -3831,13 +3831,45 @@ export class Brainy<T = any> implements BrainyInterface<T> {
    * @returns The operation to add to the caller's batch, or `null` when
    *   nothing could be done (already narrated + tracked as degraded).
    */
+  /**
+   * @description A JSON-safe view of a record bound for the metadata-index
+   * crossing. The seam's metadata is JSON-safe BY CONTRACT (a native provider
+   * serializes it; u64 ints as Number corrupt above 2^53) — but
+   * {@link resolveVerbEndpointInts} MIRRORS the resolved endpoint ints onto
+   * the verb object itself as BigInt (`verb.sourceInt`/`targetInt`), so a
+   * verb object reused as index metadata carried BigInts into
+   * JSON.stringify, which throws, aborting the whole transaction (found by
+   * the first joint pair gate). Endpoint ints ride their OWN op params on the
+   * graph legs — the metadata crossing drops every BigInt-valued top-level
+   * key instead of guessing at a lossy numeric encoding.
+   * @param metadata - The candidate index-metadata record.
+   * @returns The same object when already JSON-safe, else a shallow copy
+   *   without the BigInt-valued keys.
+   */
+  private static jsonSafeIndexMetadata(metadata: unknown): unknown {
+    if (metadata === null || typeof metadata !== 'object') return metadata
+    const rec = metadata as Record<string, unknown>
+    let hasBigint = false
+    for (const k in rec) {
+      if (typeof rec[k] === 'bigint') { hasBigint = true; break }
+    }
+    if (!hasBigint) return metadata
+    const out: Record<string, unknown> = {}
+    for (const k in rec) {
+      if (typeof rec[k] !== 'bigint') out[k] = rec[k]
+    }
+    return out
+  }
+
   private metadataIndexRetractionOp(
     id: string,
     metadata: unknown,
     context: string
   ): Operation | null {
     if (metadata) {
-      return new RemoveFromMetadataIndexOperation(this.metadataIndex, id, metadata, this.indexWriteGeneration)
+      return new RemoveFromMetadataIndexOperation(
+        this.metadataIndex, id, Brainy.jsonSafeIndexMetadata(metadata), this.indexWriteGeneration
+      )
     }
     const prov = this.metadataIndex as unknown as {
       removeEntityById?: (id: string) => Promise<void>
@@ -5114,7 +5146,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       // read above); `updatedMetadata` is the raw stored record just
       // persisted — the same shape relate()/rebuild() use to add.
       tx.addOperation(
-        new RemoveFromMetadataIndexOperation(this.metadataIndex, params.id, existing, this.indexWriteGeneration)
+        new RemoveFromMetadataIndexOperation(this.metadataIndex, params.id, Brainy.jsonSafeIndexMetadata(existing), this.indexWriteGeneration)
       )
       tx.addOperation(
         new AddToMetadataIndexOperation(this.metadataIndex, params.id, updatedMetadata, this.indexWriteGeneration)