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

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.

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.