docs: Fix audit findings and add 12 true compatibility tests

Fixes all remaining audit findings from third review:
- Documentation updated to remove 'legacy' enum value for blankLinesAfterImports
- Clarified that legacyMode controls legacy behavior, not a separate enum value
- Migration section now accurately describes legacyMode: true for migrated users
- Verified no alias command registration (only miniTypescriptHero.imports.organize)
- Verified package.json properly registers command with title and keybinding

Added 12 true compatibility tests (Task A from audit):
- TC1: Side-effect + named from same module (old crashes, new handles)
- TC2a/b: removeTrailingIndex vs merging order (legacy vs modern modes)
- TC3a/b: Idempotency tests (TypeScript and TSX files, run twice)
- TC4: Regex group precedence over keyword groups
- TC5: ignoredFromRemoval still sorts specifiers
- TC6: Within-group sorting in legacy mode (even with disableImportsSorting)
- TC7a/b: Type-only nuanced merges (legacy strips, modern preserves)
- TC8: CRLF line ending preservation
- TC9: Multiple groups with blank line separators

Test results:
- Main extension tests: 226 passing ✅
- Comparison harness tests: 159 passing ✅ (+12 new)
- Total: 385 tests passing

All critical audit findings resolved. Task B (20 edge case tests) remains
for comprehensive coverage but all core functionality is validated.

Author: JohannesHoppe

README-how-we-handle-blank-lines.md

@@ -361,18 +361,18 @@ import { A } from './a';
 export class Test {}
 ```
 
-**Mode `"legacy"`:**
-See test matrix for exact behavior (complex formula based on blanks before and after).
+**Legacy Mode (`legacyMode: true`):**
+Uses `'preserve'` behavior automatically. See test matrix for exact behavior (complex formula based on blanks before and after).
 
 ### Migration Strategy
 
 **For migrated users** (detected by settings migration):
-- Auto-set `blankLinesAfterImports` to `"legacy"`
-- Maintains exact old behavior
-- Can manually change to `"one"` anytime
+- Auto-set `legacyMode: true`
+- Maintains exact old behavior (uses `'preserve'` mode for blank lines)
+- Can manually change to `legacyMode: false` and `blankLinesAfterImports: "one"` anytime
 
 **For new users:**
-- Default `"one"` (industry standard)
+- Default `legacyMode: false` with `blankLinesAfterImports: "one"` (industry standard)
 - Clean, consistent, predictable
 
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