feat(daemon): raise trace socket receive buffer to absorb trace2 bursts

[Siddhant-K-code]

Summary

Git writes trace2 frames synchronously to the daemon's Unix socket. With small kernel socket buffers (macOS defaults to ~8 KiB), a bursty trace2 stream can fill the receive buffer and block the raw git process in write() until the daemon drains it — the back-pressure freeze class flagged for larger change-sets. This raises the trace socket's SO_RCVBUF so those bursts are absorbed in-kernel.

Shipped as a small, standalone PR so it can land independently of the socket-drain health work.

What changed (Unix only; Windows uses named pipes and is untouched):

// new: SO_RCVBUF helper, applied on the trace listener AND each accepted conn
set_socket_recv_buffer(fd, trace_socket_recv_buffer_bytes())  // setsockopt(SO_RCVBUF)

trace_listener_loop_actor:
  listener = bind(trace.sock)
+ set_trace_listener_recv_buffer(&listener)   // accepted conns inherit it early
  for stream in listener.incoming():
+     set_trace_socket_recv_buffer(&stream)    // backstop per-connection

• Default target is a conservative 512 KiB (TRACE_SOCKET_RECV_BUFFER_BYTES), and is overridable at runtime via GIT_AI_TRACE_SOCKET_RECV_BUFFER_BYTES so it can be ramped toward 1 MiB without a code change. 0 disables the bump.
• The constant and all setsockopt code are #[cfg(not(windows))]; Windows trace ingestion uses named pipes, so there is nothing to gate there.

Important: this is a mitigation, not a guarantee. A larger buffer absorbs bursty trace output, but any finite buffer can still fill if the daemon genuinely stops draining — that drain-stall detection is handled separately (ping/sentinel health check, #1735).

Testing

• set_socket_recv_buffer raises a socket's SO_RCVBUF toward the target. The assertion is robust to Linux clamping SO_RCVBUF to net.core.rmem_max (CI runners default to ~208 KiB): it passes if the buffer reaches the target or grows past the socket's default baseline.
• A zero target is a verified no-op (does not error, does not shrink the existing buffer).
• cargo fmt, cargo clippy --all-targets -- -D warnings, cargo doc all clean (Rust 1.93.0, matching CI).

Link to Devin session: https://app.devin.ai/sessions/a9931f84ae914fcc82663a2e52106bd5
Requested by: @Siddhant-K-code

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✅ Devin Review: No Issues Found

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