fix(mediation): add ACK handshake to drain buffer before sending stdio fds

[kipz]

Problem

Commands with large environments fail with:

nono-shim: failed to send stdio fds: Message too long (os error 40)

This is EMSGSIZE. The root cause is a race between the JSON write and the SCM_RIGHTS sendmsg: the shim sends the full process environment in the JSON request (which can be 6–9 KB), filling the macOS UDS receive buffer (~8 KB default). When the shim immediately calls sendmsg with the SCM_RIGHTS control message, there is no contiguous buffer space left for the ancillary data — even with all three fds bundled into a single sendmsg.

Fix

Add a 1-byte ACK handshake between the JSON read and the fd receive:

1. Server reads the JSON body (buffer now drained)
2. Server sends 0x06 ACK
3. Shim reads ACK, then calls sendmsg

This guarantees the buffer is empty at the point the SCM_RIGHTS message is sent.

Protocol change — shim and server must ship together.

Updated protocol:

1. Request:  u32 length || JSON body   (unchanged)
2. ACK:      0x06 from server          (new)
3. SCM_RIGHTS sendmsg — stdio fds      (unchanged)
4. Response: u32 length || JSON body   (unchanged)

Test

Added ack_handshake_round_trips to nono-shim — spins up a mock server thread that follows the new protocol, verifies the SCM_RIGHTS transfer succeeds, and checks the exit code.