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>

addons/addon-serialize/src/SerializeAddon.ts

@@ -7,7 +7,8 @@
 
 import type { IBuffer, IBufferCell, IBufferRange, ITerminalAddon, Terminal } from '@xterm/xterm';
 import type { IHTMLSerializeOptions, SerializeAddon as ISerializeApi, ISerializeOptions, ISerializeRange } from '@xterm/addon-serialize';
-import { IAttributeData, IColor } from 'common/Types';
+import { IColor } from 'common/Types';
+import { IAttributeData } from 'common/buffer/Types';
 import { DEFAULT_ANSI_COLORS } from 'browser/Types';
 import { UnderlineStyle } from 'common/buffer/Constants';