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/REARCHITECTURE.md

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+# Common folder rearchitecture
+
+Layered layout for `src/common/` as proposed in [issue #5963](https://github.com/xtermjs/xterm.js/issues/5963).
+
+## Dependency graph (target)
+
+```mermaid
+flowchart BT
+  subgraph platform["Platform layer"]
+    direction LR
+    base["common/base"]
+    encoding["common/encoding"]
+    data["common/data"]
+    buffer["common/buffer"]
+    parser["common/parser"]
+    input["common/input"]
+
+    encoding --> base
+    data --> base
+    buffer --> encoding
+    buffer --> data
+    parser --> buffer
+    input --> parser
+  end
+
+  subgraph integration["Integration layer"]
+    direction LR
+    services["common/services"]
+    terminal["common/terminal"]
+    terminal --> services
+  end
+
+  subgraph api_layer["API layer"]
+    public["common/public"]
+  end
+
+  integration --> platform
+  api_layer --> integration
+```
+
+## Package contents
+
+| Project | Role | Notable sources |
+| --- | --- | --- |
+| `common/base` | Core utilities (lifecycle, events, platform) | `Async`, `Event`, `Lifecycle`, `Platform`, `StringBuilder`, `Version` |
+| `common/encoding` | UTF conversion | `TextDecoder` (moved from `input/`) |
+| `common/data` | Static terminal data | `Charsets`, `EscapeSequences` |
+| `common/buffer` | Screen buffer | `Buffer`, `BufferLine`, … |
+| `common/parser` | Escape sequence parser | `EscapeSequenceParser`, … |
+| `common/input` | Keyboard / write path helpers | `Keyboard`, `WriteBuffer`, … |
+| `common/services` | DI services and shared types | `Services`, `Types`, `CircularList`, … |
+| `common/terminal` | Core terminal + input handler | `CoreTerminal`, `InputHandler`, `WindowsMode` |
+| `common/public` | Public API adapters | `AddonManager`, buffer API views |
+
+## TypeScript projects
+
+- **`common/tsconfig.json`** — composite build for all layers (resolves existing `buffer` ↔ `services` imports).
+- **`common/<layer>/tsconfig.json`** — per-layer config extending the root (`noEmit`) for tooling and future split into referenced composite projects.
+
+Enforcing one-way project references at compile time (separate `out` per layer) is follow-up work once `Types` and buffer service interfaces are decoupled.