fix(metadata): the indexable-array bound is 256 — a keyword list is not a vector

64 cleared tags, authors and labels, but not the shape that actually turns up
in production metadata: a long keyword or participant list. 256 clears those
and still refuses every embedding this engine will ever meet — the narrowest
model it ships is 384-dimensional, so the two populations still do not overlap
and nobody has to tune anything. A vector parked in metadata throws by name;
a 200-keyword list writes and indexes.

The number lives in ONE place, `MAX_INDEXED_ARRAY_LENGTH`, and every message,
warning and pin derives it from there. Two pins still carried a literal:
metadata-vector-exclusion refused an array of exactly 100 — which sits UNDER
the new bound, so the case would have asserted a refusal that no longer
happens — and the array-bound suite named "all 64 elements" in a title and
picked its middle element as a hardcoded 't31'. Both derive from the constant
now, so the pins follow it wherever it goes rather than silently inverting the
next time it moves.

tests/unit/utils/metadataIndex-array-bound.test.ts

@@ -15,9 +15,10 @@
  * and the caller had no way to tell that from "no row matches". Eleven tags is
  * not an exotic shape; the eleventh tag made the row invisible.
  *
- * THE LAW. Arrays of scalars index up to {@link MAX_INDEXED_ARRAY_LENGTH} = 64,
+ * THE LAW. Arrays of scalars index up to {@link MAX_INDEXED_ARRAY_LENGTH},
  * hardcoded (the zero-config law: no knob), which clears every legitimate
- * multi-value field and stays far below any embedding width. Above it the WRITE
+ * multi-value field — tags, authors, keyword lists — and stays below the
+ * narrowest embedding this engine meets (384 dimensions). Above it the WRITE
  * IS REFUSED by name — `MetadataArrayTooLargeError`, carrying the field, the
  * length and the bound — at `add`, `update`, `relate` and `updateRelation`
  * alike. Nothing is skipped in silence.
@@ -65,7 +66,7 @@ describe('the indexable-array bound', () => {
       }
     })
 
-    it('indexes right up to the bound — all 64 elements', async () => {
+    it('indexes right up to the bound — every element of it', async () => {
       await brain.add({
         id: 'at-bound',
         data: 'a row at the bound',
@@ -74,8 +75,9 @@ describe('the indexable-array bound', () => {
         vector: []
       })
 
-      // The first, the last, and one in the middle.
-      for (const tag of ['t0', `t${MAX_INDEXED_ARRAY_LENGTH - 1}`, 't31']) {
+      // The first, the last, and one in the middle — all derived from the
+      // bound, so the case follows the constant wherever it moves.
+      for (const tag of ['t0', `t${MAX_INDEXED_ARRAY_LENGTH - 1}`, `t${Math.floor(MAX_INDEXED_ARRAY_LENGTH / 2)}`]) {
         const hits = await brain.find({ where: { tags: tag }, limit: 10 } as any)
         expect(hits.map((r: any) => r.id)).toContain(resolveEntityId('at-bound'))
       }