fix: find()/stats() correctness + Cortex compat (BR-FIND-WHERE-ZERO, BR-DEFENSIVE-INTERFACE)

Two production correctness defects fixed together so consumers can land
one upgrade.

BR-FIND-WHERE-ZERO — find() returned [] and stats() reported 0 entities
for any workspace whose data was written after the 7.20.0 column-store
refactor. Root cause: getStats() and the getIds() fallback still read
from the deleted sparse-index path. Separately, BaseStorage.getNounType()
was hardcoded to return 'thing', poisoning type-statistics.json with
every noun attributed to that bucket.

  - MetadataIndex.getStats() reads from ColumnStore + idMapper.
  - MetadataIndex.getIdsFromChunks() throws BrainyError(FIELD_NOT_INDEXED)
    when neither store has the field. getIdsForFilter() catches per
    clause, logs once, returns [].
  - BaseStorage.nounTypeByIdCache populated in saveNounMetadata_internal
    and consumed in saveNoun_internal. flushCounts() now persists the
    Uint32Array counters too, so readers see fresh per-type counts.
  - Self-heal at init: loadTypeStatistics() auto-rebuilds when the
    poisoned signature is detected. rebuildTypeCounts() is now public for
    use from `brainy inspect repair`.
  - Dead state removed: dirtyChunks, dirtySparseIndices,
    flushDirtyMetadata().

BR-DEFENSIVE-INTERFACE — 7.21.0 called supportsMultiProcessLocking()
unconditionally, crashing on older Cortex storage adapters that predate
the method. New hasStorageMethod(name) helper gates every new-method
call site. Older adapter triggers a one-line warning at init pointing
at the recommended plugin version.

Tests:
  - new tests/integration/find-where-zero.test.ts (7 cases)
  - new tests/integration/cortex-compat.test.ts (5 cases)
  - multi-process-safety.test.ts updated to enforce correct counts
  - 1329/1329 unit + 24/24 new integration tests passing

src/errors/brainyError.ts

@@ -3,7 +3,14 @@
  * Provides better error classification and handling
  */
 
-export type BrainyErrorType = 'TIMEOUT' | 'NETWORK' | 'STORAGE' | 'NOT_FOUND' | 'RETRY_EXHAUSTED' | 'VALIDATION'
+export type BrainyErrorType =
+  | 'TIMEOUT'
+  | 'NETWORK'
+  | 'STORAGE'
+  | 'NOT_FOUND'
+  | 'RETRY_EXHAUSTED'
+  | 'VALIDATION'
+  | 'FIELD_NOT_INDEXED'
 
 /**
  * Custom error class for Brainy operations
@@ -99,6 +106,25 @@ export class BrainyError extends Error {
         )
     }
 
+    /**
+     * Create a "field is not indexed" error. Thrown by metadata-index reads
+     * when a `where` clause names a field that has neither a column-store
+     * entry nor a sparse-index entry. Callers in `find()` evaluation catch
+     * this, translate the offending clause to an empty result, and log so
+     * the silent-empty behavior is replaced with a loud one. Use
+     * `brain.explain({ where: {...} })` to discover this before running.
+     */
+    static fieldNotIndexed(field: string): BrainyError {
+        return new BrainyError(
+            `Field "${field}" is not indexed. find()/where will not match any entities. ` +
+            `Likely causes: (1) the writer registered the field in memory but has not flushed; ` +
+            `(2) the field name is mistyped; (3) no entity has ever held this field. ` +
+            `Run brain.explain({ where: { ${field}: ... } }) for the diagnostic.`,
+            'FIELD_NOT_INDEXED',
+            false
+        )
+    }
+
     /**
      * Create a validation error
      */