fix: aggregation surfaces materialize/state-load failures loudly

The debounced materialization caught its failure with an empty `catch (() => {})`,
silently leaving the materialized Measurement entity stale; the aggregation-index
init() state-load failure was swallowed the same way, leaving aggregates reading
empty with no signal. Both are non-fatal (values rebuild via backfill-on-query),
but a silent stale/empty read violates loud-errors-never-quiet-losses. Both now
emit a loud warning naming the affected group / cause.

src/aggregation/materializer.ts

@@ -14,6 +14,7 @@ import type {
   AggregateMetricDef
 } from '../types/brainy.types.js'
 import { serializeGroupKey } from './AggregationIndex.js'
+import { prodLog } from '../utils/logger.js'
 
 /**
  * Callback interface for the materializer to create/update Brainy entities.
@@ -86,8 +87,15 @@ export class AggregateMaterializer {
     if (existing) clearTimeout(existing)
 
     this.debounceTimers.set(key, setTimeout(() => {
-      this.materializeOne(key).catch(() => {
-        // Non-fatal — materialization is derived data
+      this.materializeOne(key).catch((err) => {
+        // Materialization is derived data (rebuildable via backfill-on-query), so
+        // a failure is non-fatal — but it leaves the materialized Measurement
+        // entity STALE. Surface it loudly rather than swallow it silently.
+        prodLog.warn(
+          `[Aggregation] Failed to materialize aggregate group '${key}': ` +
+          `${(err as Error).message}. The materialized value is stale until the ` +
+          `next successful materialization or a backfill-on-query.`
+        )
       })
     }, debounceMs))
   }