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/common/input/Win32InputMode.ts

@@ -14,7 +14,7 @@
  *   Rc: Repeat count (usually 1)
  */
 
-import { IKeyboardEvent, IKeyboardResult, KeyboardResultType } from 'common/Types';
+import { IKeyboardEvent, IKeyboardResult, KeyboardResultType } from 'common/services/Types';
 import { C0 } from 'common/data/EscapeSequences';
 
 /**