refactor: extract helpers, rename for clarity, fix types, remove dead code

- Add fail() helper (always throws) replacing 7 instances of inline error handling
- Rename strip-asserts → code-transforms, cleanDiffMeta → cleanMeta
- Extract handleDiff/handleDiffStep from visitor with shared ctx object
- Extract parseFlags for DRY flag parsing
- Fix PRESET_MARKERS type (RegionMarker per key, not RegionMarker[])
- Remove dead code branches from simplified fail() flow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: patricker

index.d.mts

@@ -66,9 +66,9 @@ export declare const PRESET_CLEAN: {
 
 // Preset region markers (grouped by syntax)
 export declare const PRESET_MARKERS: {
-  css: RegionMarker[];
-  html: RegionMarker[];
-  sql: RegionMarker[];
+  css: RegionMarker;
+  html: RegionMarker;
+  sql: RegionMarker;
 };
 
 // Default values (unions of all presets)