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.

src/errors/brainyError.ts

@@ -412,18 +412,23 @@ export class MigrationInProgressError extends BrainyError {
  * embedding parked in the metadata bag would mint 384 postings for one row.
  * The bound exists to keep that out of the index.
  *
- * 64 is hardcoded on purpose (the zero-config law: no knob). It sits far above
+ * 256 is hardcoded on purpose (the zero-config law: no knob). It sits far above
  * every legitimate multi-value field the engine has seen — tags, authors,
- * categories, labels, participant lists — and far below any real embedding
- * width, so the two populations do not overlap and no caller has to tune it.
+ * categories, labels, keyword lists, participant lists — and still below the
+ * narrowest embedding this engine will ever meet (384 dimensions, the smallest
+ * model it ships), so the two populations do not overlap and no caller has to
+ * tune it. A vector parked in metadata is refused; a long keyword list is not.
  *
  * It replaces a limit of 10 that was applied SILENTLY: a row whose `tags` array
  * held eleven entries had that field skipped entirely and dropped out of every
  * filtered search on it, with no error, no warning and no way to tell the
  * difference from "no row matches". A rule this consequential is a law with a
  * name and a refusal, not a `continue`.
+ *
+ * This is the ONE place the number lives. Every message, warning, doc line and
+ * pin derives it from here — never a literal.
  */
-export const MAX_INDEXED_ARRAY_LENGTH = 64
+export const MAX_INDEXED_ARRAY_LENGTH = 256
 
 /**
  * A metadata field carries an array longer than {@link MAX_INDEXED_ARRAY_LENGTH}.