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

从 lib/src/lib/terminal-transfer.ts 开始，运行 lib/src/lib/terminal-transfer.test.ts 中指定的往返测试，然后阅读 docs/specs/transport.md 和 docs/specs/webgl-text.md，了解传输和版本锁步要求。完成的标准是 addon-serialize 发出鼠标编码，固定的 @xterm beta 版本得到升级，移除私有状态读取，并且回归测试通过。

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.