Refactor decoration line index into DecorationLineCache

Extract the per-line lookup index from DecorationService into an exported
DecorationLineCache class in the same module, with DecorationService listed
first for readability.

Optimize buffer insert/delete maintenance to shift map keys in O(unique
indexed lines) instead of full re-indexing every affected decoration. Only
multi-line decorations that span the mutation point get a full re-index.

Add a direct DecorationLineCache unit test in DecorationService.test.ts.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/common/services/DecorationService.test.ts

@@ -4,7 +4,7 @@
  */
 
 import { assert } from 'chai';
-import { DecorationService } from './DecorationService';
+import { DecorationLineCache, DecorationService } from './DecorationService';
 import { IMarker } from 'common/Types';
 import { Disposable } from 'common/Lifecycle';
 import { Emitter } from 'common/Event';
@@ -149,6 +149,13 @@ describe('DecorationService', () => {
     });
   });
 
+  describe('DecorationLineCache', () => {
+    it('should return undefined for lines with no indexed decorations', () => {
+      const cache = new DecorationLineCache();
+      assert.isUndefined(cache.getDecorationsOnLine(0));
+    });
+  });
+
   describe('line index maintenance', () => {
     it('should keep lookups correct after buffer trim', () => {
       const bufferService = new MockBufferService(80, 5, new MockOptionsService({ scrollback: 0 }));