fix: extraction, multi-hop traversal, and aggregate result shape (BR-ADV-FEATURES-BUN)

Three advanced-API correctness fixes, all reproducible on Node (not Bun-specific):

- Entity/concept extraction returned []. SmartExtractor combined agreeing signals
  with a weighted sum compared against an absolute 0.60 gate, so a confident
  low-weight signal lost selection to a mediocre high-weight one that then failed
  the gate, dropping the whole result. Select and gate on a normalized weighted
  average instead. The public `confidence` option now controls the threshold (was
  a dead hardcoded 0.60), and the embedding-signal timeout is raised 100ms -> 2000ms
  so the neural signal is not silently dropped on slower runtimes.

- Multi-hop find({ connected }) returned only the 1-hop neighbour. executeGraphSearch
  ignored depth/via; it now delegates to the depth-aware neighbors() BFS.

- find({ aggregate }) hid groupKey/metrics/count under .metadata, so callers
  expecting AggregateResult saw empty rows. Expose those fields at the top level.

Adds real-embedding regression tests in tests/integration/advanced-apis-regression.test.ts.

src/types/brainy.types.ts

@@ -122,6 +122,14 @@ export interface Result<T = any> {
   semanticScore?: number      // Semantic similarity score (0-1)
   matchSource?: 'text' | 'semantic' | 'both'  // Where this result came from
   rrfScore?: number           // Raw RRF fusion score (for advanced ranking analysis)
+
+  // Aggregation: present only on rows returned by find({ aggregate }). These mirror the
+  // documented AggregateResult shape so callers can read group/metric data at the top level
+  // instead of digging into `metadata`. (The same values are also flattened into `metadata`
+  // for backward compatibility.)
+  groupKey?: Record<string, string | number>   // Group-by key values for this aggregate row
+  metrics?: Record<string, number>             // Computed metric values (sum/avg/min/max/count)
+  count?: number                               // Total entity count in this group
 }
 
 /**