fix(transact): metadata-index ops take their JSON-safe view at the crossing, not at construction

transact()'s delete legs (direct unrelate and the noun-remove cascade) hand
the SAME verb object to the graph-retraction op and the metadata-retraction
op. The metadata leg sanitized at PLAN time, when the verb was still clean,
so the wrap returned the same reference — then the graph op's execute-time
endpoint resolution (deliberately deferred for same-batch forward refs)
mirrored BigInt sourceInt/targetInt onto the shared object, and the metadata
op crossed the seam with them. A strict provider rightly refuses that
crossing, so every transact-wrapped edge delete aborted; direct unrelate()
resolves ints at build time, before its sanitize, which is why no existing
gate saw it.

The JSON-safe view now lives in a shared leaf (utils/jsonSafeIndexMetadata)
and is applied INSIDE AddToMetadataIndexOperation and
RemoveFromMetadataIndexOperation at execute and rollback time — the one
place no plan-vs-execute ordering can bypass. Pins: the fleet repro, the
cascade shape, a mixed batch, and unit pins that mutate the entity after
construction against a strict seam (5 red before, 5 green after).

src/utils/jsonSafeIndexMetadata.ts

@@ -0,0 +1,47 @@
+/**
+ * @module utils/jsonSafeIndexMetadata
+ * @description The metadata-index crossing's JSON-safety law, as a leaf
+ * function both the coordinator and the transaction operations share.
+ *
+ * The seam's metadata is JSON-safe BY CONTRACT (a native provider serializes
+ * it; u64 ints as Number corrupt above 2^53) — but `resolveVerbEndpointInts`
+ * MIRRORS the resolved endpoint ints onto the verb object itself as BigInt
+ * (`verb.sourceInt`/`targetInt`), so a verb object reused as index metadata
+ * carries BigInts into JSON.stringify, which throws, aborting the whole
+ * transaction. 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.
+ *
+ * WHY THIS IS A LEAF MODULE, ENFORCED AT THE CROSSING: sanitizing only at
+ * operation-construction time is not enough. `transact()`'s delete legs pass
+ * the SAME verb object to both the graph-retraction op (whose endpoint-int
+ * thunk deliberately resolves at EXECUTE time, for same-batch forward refs)
+ * and the metadata-retraction op. At plan time the verb is still clean, so a
+ * plan-time sanitize returns the same reference — then the graph op executes
+ * first, mirrors the BigInt ints onto the shared object, and the metadata op
+ * crosses the seam with them (found by the first fleet adoption of the native
+ * pair: every transact-wrapped edge delete aborted). The crossing itself is
+ * the only place ordering cannot bypass.
+ */
+
+/**
+ * A JSON-safe view of a record bound for the metadata-index crossing.
+ *
+ * @param metadata - The candidate index-metadata record.
+ * @returns The same object when already JSON-safe, else a shallow copy
+ *   without the BigInt-valued keys.
+ */
+export function 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
+}