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

frontend/src/routes/migration/+page.svelte

@@ -1,6 +1,6 @@
 <!-- [DEF:frontend/src/routes/migration/+page.svelte:Module] -->
 <!--
-@TIER: CRITICAL
+@COMPLEXITY: 5
 @SEMANTICS: migration, dashboard, environment, selection, database-replacement, dry-run, task-resume
 @PURPOSE: Main migration dashboard page for environment selection, dry-run validation, and migration execution.
 @LAYER: UI

frontend/src/routes/migration/mappings/+page.svelte

@@ -1,6 +1,6 @@
 <!-- [DEF:frontend/src/routes/migration/mappings/+page.svelte:Module] -->
 <!--
-@TIER: CRITICAL
+@COMPLEXITY: 5
 @SEMANTICS: migration-mappings, environment-selection, fuzzy-matching, persistence-contract, ux-state-machine
 @PURPOSE: Render and orchestrate mapping management UI for source/target environments with backend persistence.
 @LAYER: UI