refactor(semantics): migrate legacy @TIER to @COMPLEXITY annotations

- Replaced @TIER: TRIVIAL with @COMPLEXITY: 1
- Replaced @TIER: STANDARD with @COMPLEXITY: 3
- Replaced @TIER: CRITICAL with @COMPLEXITY: 5
- Manually elevated specific critical/complex components to levels 2 and 4
- Ignored legacy, specs, and node_modules directories
- Updated generated semantic map

backend/tests/core/test_migration_engine.py

@@ -1,6 +1,6 @@
 # [DEF:backend.tests.core.test_migration_engine:Module]
 #
-# @TIER:      STANDARD
+# @COMPLEXITY: 3
 # @PURPOSE:   Unit tests for MigrationEngine's cross-filter patching algorithms.
 # @LAYER:     Domain
 # @RELATION:  VERIFIES -> backend.src.core.migration_engine