Workspace transfer reads xterm private state because addon-serialize drops mouse encoding

## Problem

When a Workspace moves between standalone windows, each terminal is rebuilt in the target from `@xterm/addon-serialize` output (`serializeTransferTerminal` in `lib/src/lib/terminal-transfer.ts`). The pinned addon (`@xterm/addon-serialize` `0.15.0-beta.301`, with `@xterm/xterm` `6.1.0-beta.304`) serializes the mouse **tracking** mode (`?1000` / `?1002` / `?1003`) but not the mouse **encoding**: SGR (`CSI ? 1006 h`) or SGR-pixels (`CSI ? 1016 h`).

Without it, a full-screen program that turned on SGR mouse reporting (vim, tmux, htop, …) arrives in the new window with xterm's default encoding. From then on its mouse reports are misparsed, and clicks past column 223 cannot be encoded at all.

## Current workaround (#630)

`serializeTransferTerminal` reads xterm's private `terminal._core.mouseStateService.activeEncoding` and appends `\x1b[?1006h` or `\x1b[?1016h` after the serialized buffer. `docs/specs/transport.md` → "Transferring a Workspace" requires the encoding to survive the move, and `lib/src/lib/terminal-transfer.test.ts` pins it against real xterm parsing:

- `preserves mouse tracking and encoding %i through real xterm parsing`
- `does not resurrect encoding after reset %j`
- `still transfers the buffer if private mouse state is unavailable`

The weakness is the private field. If an xterm bump renames or moves `mouseStateService` / `activeEncoding`, the lookup returns nothing and the buffer transfers without the encoding. The round-trip tests would fail that bump rather than let it regress silently, but the code still reaches into xterm internals.

## Proposed fix

1. Upstream: have `addon-serialize` emit the active mouse encoding alongside the tracking mode (xtermjs/xterm.js, `addons/addon-serialize`).
2. Bump the pinned `@xterm/*` betas, respecting the version lockstep in `docs/specs/webgl-text.md`.
3. Delete the private-state read in `serializeTransferTerminal`; keep the round-trip tests as the regression pin.

Start in lib/src/lib/terminal-transfer.ts and run the named round-trip tests in lib/src/lib/terminal-transfer.test.ts, then read docs/specs/transport.md and docs/specs/webgl-text.md for transfer and version-lockstep requirements. Done means addon-serialize emits mouse encoding, the pinned @xterm betas are bumped, the private-state read is removed, and the regression tests pass.

Written by the indexing model from the issue text.