Restructure common into layered folders (issue #5963)

Reorganize src/common into platform (base, encoding, data, buffer,
parser, input), integration (services, terminal), and public API layers.

- Move core utilities to common/base/
- Move TextDecoder to common/encoding/
- Move Types, CircularList, Color, TaskQueue, SortedList to common/services/
- Move CoreTerminal, InputHandler, WindowsMode to common/terminal/
- Define ICharset in common/data/Charsets.ts (re-exported from Types)
- Update imports across src/, addons/, and test/
- Add per-layer tsconfig.json files and REARCHITECTURE.md
- Root common/tsconfig.json remains the composite build entry

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

src/browser/renderer/shared/TextBlinkStateManager.test.ts

@@ -5,9 +5,9 @@
 
 import { assert } from 'chai';
 import { TextBlinkStateManager } from 'browser/renderer/shared/TextBlinkStateManager';
-import { MockOptionsService } from 'common/TestUtils.test';
+import { MockOptionsService } from 'common/terminal/TestUtils.test';
 import type { ICoreBrowserService } from 'browser/services/Services';
-import { Emitter } from 'common/Event';
+import { Emitter } from 'common/base/Event';
 
 class FakeWindow {
   public nextId = 1;

src/browser/renderer/shared/TextBlinkStateManager.ts

@@ -4,7 +4,7 @@
  */
 
 import { ICoreBrowserService } from 'browser/services/Services';
-import { Disposable, toDisposable } from 'common/Lifecycle';
+import { Disposable, toDisposable } from 'common/base/Lifecycle';
 import { IOptionsService } from 'common/services/Services';
 
 export class TextBlinkStateManager extends Disposable {

src/browser/renderer/shared/Types.ts

@@ -5,8 +5,8 @@
 
 import { Terminal } from '@xterm/xterm';
 import { ITerminal } from 'browser/Types';
-import { IDisposable } from 'common/Types';
-import type { IEvent } from 'common/Event';
+import { IDisposable } from 'common/services/Types';
+import type { IEvent } from 'common/base/Event';
 
 export interface IDimensions {
   width: number;