feat(update-weight): add mooncake transfer engine transport

[Vincent-Bo-ali]

Summary

This PR adds an experimental Mooncake TransferEngine transport for full-weight sync to slime-managed SGLang engines.

Instead of forming a trainer-engine NCCL update group or writing full checkpoints to a shared filesystem, slime now can:

1. Ask each SGLang engine to initialize a local Mooncake receiver buffer.
2. Write each full-weight bucket into that receiver buffer through Mooncake TransferEngine.
3. Send a manifest to SGLang so the engine loads tensors from its local receiver buffer.
4. Destroy the receiver cleanly when the rollout engine disconnects.

This path is intentionally scoped to --update-weight-mode full and slime-managed SGLang engines.

User-Facing Interface

New transport option:

--update-weight-mode full
--update-weight-transport mooncake
--mooncake-metadata-server P2PHANDSHAKE
--mooncake-protocol tcp
--mooncake-buffer-size $((8 * 1024 * 1024 * 1024))
--mooncake-buffer-count 1