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/integration/metadata-vector-exclusion.test.ts

@@ -161,7 +161,8 @@ describe('Metadata Vector Exclusion Fix', () => {
     // silence at a bound of 10 — the field simply vanished from the index and
     // the row dropped out of every `where` on it, indistinguishably from "no
     // row matches". The bound is now MAX_INDEXED_ARRAY_LENGTH and it REFUSES.
-    const largeArray = Array.from({ length: 100 }, (_, i) => `item${i}`)
+    const overTheBound = MAX_INDEXED_ARRAY_LENGTH + 1
+    const largeArray = Array.from({ length: overTheBound }, (_, i) => `item${i}`)
 
     const err = await brainy
       .add({
@@ -176,7 +177,7 @@ describe('Metadata Vector Exclusion Fix', () => {
 
     expect(err).toBeInstanceOf(MetadataArrayTooLargeError)
     expect(err.field).toBe('items')
-    expect(err.length).toBe(100)
+    expect(err.length).toBe(overTheBound)
     expect(err.limit).toBe(MAX_INDEXED_ARRAY_LENGTH)
 
     // Nothing was indexed from the refused write — no 'items' field, and above