feat(8.0): reserved-field enforcement — reservedFieldPolicy defaults to throw

An untyped (JS) caller that smuggles a Brainy-reserved field (confidence,
weight, subtype, visibility, service, createdBy, noun/verb, data, createdAt,
updatedAt, _rev) inside a write-path metadata bag previously got a silent
remap-or-drop — a class of bug where confidence-evolution writes no-oped for
weeks before being caught on read-back. 8.0 closes this with no silent failures.

- New BrainyConfig.reservedFieldPolicy: 'throw' | 'warn' | 'remap' (default 'throw').
  'throw' rejects the write naming every offending key + its correct write path;
  'warn' remaps with a one-shot per-key warning; 'remap' is the legacy silent path.
- Central enforceReservedPolicy gate wired into all four remap methods (add,
  update, relate, updateRelation) so live calls AND their transact()/with()
  mirrors honor it. Single-source reservedWritePath guidance shared by throw+warn.
- 'warn' now warns for EVERY reserved key (closes the gap where only
  system-managed fields warned). Dead warnDropped* helpers removed.
- Import pipeline migrated to route reserved values (confidence/weight/subtype)
  through dedicated params and strip reserved keys from extractor/customMetadata
  bags via the canonical split*MetadataRecord helpers — imports no longer trip
  the default throw.
- Tests: new reservedFieldPolicy matrix (throw/warn/remap across every write
  path + transact); remap-correctness suite reframed as opt-in 'remap'; shared
  test-factory no longer emits reserved keys in custom metadata.

tests/api/performance-benchmarks.test.ts

@@ -201,7 +201,8 @@ describe('Performance Benchmarks - SLA Validation', () => {
         const start = performance.now()
         await brainy.update({
           id,
-          metadata: { version: 2, updatedAt: Date.now() }
+          // `updatedAt` is system-managed — Brainy sets it on every write.
+          metadata: { version: 2 }
         })
         const latency = performance.now() - start
         benchmark.recordOperation(latency)