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/common/TestUtils.test.ts

@@ -6,9 +6,9 @@
 import { IBufferService, ICoreService, ILogService, IOptionsService, ITerminalOptions, IMouseStateService, ICharsetService, UnicodeCharProperties, UnicodeCharWidth, IUnicodeService, IUnicodeVersionProvider, LogLevelEnum, IDecorationService, IInternalDecoration, IOscLinkService, type IBufferResizeEvent } from './services/Services';
 import { UnicodeService } from './services/UnicodeService';
 import { DEFAULT_OPTIONS } from './services/OptionsService';
-import { IBufferSet, IBuffer } from './buffer/Types';
+import { IAttributeData, IBuffer, IBufferLine, IBufferSet, IExtendedAttrs } from './buffer/Types';
 import { BufferSet } from './buffer/BufferSet';
-import { IDecPrivateModes, ICoreMouseEvent, CoreMouseEventType, ICharset, IModes, IAttributeData, IOscLinkData, IDisposable, IBufferLine, IExtendedAttrs } from './Types';
+import { IDecPrivateModes, ICoreMouseEvent, CoreMouseEventType, ICharset, IModes, IOscLinkData, IDisposable } from './Types';
 import { UnicodeV6 } from './input/UnicodeV6';
 import { IDecorationOptions, IDecoration } from '@xterm/xterm';
 import { Emitter, type IEvent } from './Event';