semantics: complete DEF-to-region migration, fix regressions

- Convert legacy [DEF🆔Type] anchors to #region/#endregion across 329 files
- Reinstate _normalize_timestamp_value in sql_generator.py
- Fix MarkerLogger→logger migration in events.py (molecular CoT markers)
- Fix dataset_review orchestrator dependencies (_build_execution_snapshot)
- Fix config_manager stale-record deletion (moved to save path only)
- Add 77 missing [/DEF:] closers in 5 unbalanced test files
- Update assistant_chat.integration.test.js for #region format
- Apply molecular-cot-logging markers (REASON/REFLECT/EXPLORE) via logger.* methods

backend/src/core/config_models.py

@@ -1,6 +1,6 @@
 # #region ConfigModels [C:3] [TYPE Module] [SEMANTICS config, models, pydantic]
 # @BRIEF Defines the data models for application configuration using Pydantic.
-# @LAYER Core
+# @LAYER: Core
 # @RELATION IMPLEMENTS -> [CoreContracts]
 # @RELATION IMPLEMENTS -> [ConnectionContracts]
 
@@ -71,7 +71,7 @@ class CleanReleaseConfig(BaseModel):
 
 # #region FeaturesConfig [C:1] [TYPE DataClass]
 # @BRIEF Top-level feature flags that toggle entire project features on/off.
-# @RATIONALE Features are read from environment variables on bootstrap and persisted in DB.
+# @RATIONALE: Features are read from environment variables on bootstrap and persisted in DB.
 #            DB is source of truth after initial bootstrap; env vars only seed defaults.
 class FeaturesConfig(BaseModel):
     dataset_review: bool = True