Colocate buffer types in buffer/Types.ts

The import/no-cycle refactor had moved IBuffer, IBufferSet, and related
cell/line types into src/common/Types.ts, with buffer/Types.ts only
re-exporting them. That inverted the natural module boundary and made
buffer code depend on the top-level types hub.

Define buffer-owned types in buffer/Types.ts instead:
- CharData, IExtendedAttrs, IAttributeData, ICellData, IBufferLine
- IMarker, IBuffer, IBufferSet, BufferIndex

Restore UnderlineStyle to buffer/Constants.ts (used by IExtendedAttrs)
and remove those definitions from Types.ts.

buffer/Types.ts imports only what it needs from outside the buffer layer:
ICircularList and ICharset from Types, IDisposable from Lifecycle,
IEvent from Event, and UnderlineStyle from Constants. Types.ts does not
import buffer/Types.ts, so the previous Types <-> buffer cycle stays broken.

Update imports across common, browser, headless, addons, and tests to use
common/buffer/Types for buffer types rather than common/Types.

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

src/browser/Terminal.test.ts

@@ -9,7 +9,7 @@ import { assert } from 'chai';
 import { DEFAULT_ATTR_DATA } from '../common/buffer/BufferLine';
 import { CellData } from '../common/buffer/CellData';
 import { MockUnicodeService, createCellData } from '../common/TestUtils.test';
-import { IMarker } from '../common/Types';
+import { IMarker } from '../common/buffer/Types';
 
 const INIT_COLS = 80;
 const INIT_ROWS = 24;