Retry AVF runner connects refused by a full backlog

[einar-kindly]

Problem

After upgrading to macOS 27, agv start on an AVF VM with 7 [[forwards]] logged:

WARN agv::vm: could not resolve SSH endpoint for managed SSH config vm=kindly
error=failed to connect to AVF runner socket .../avf-control.sock: Connection refused (os error 61)

The VM itself was fine and agv ssh worked, but the managed ssh_config was left empty, so plain ssh <vm> (and IDE remote access) didn't.

Cause

At the end of agv start, agv spawns one forward supervisor per forward and then calls update_ssh_config. All of them call the runner's status op at roughly the same moment: 8 connects. The runner listened with listen(fd, 4), and macOS answers a full unix-socket backlog with an immediate ECONNREFUSED rather than queueing the connect (measured on macOS 27: a backlog of N accepts exactly N pending connects). The supervisors retry on their own, so forwards came up, but update_ssh_config is one-shot and lost the race.

Why it surfaced now isn't proven: likely a timing shift or a change in how the kernel counts the backlog on macOS 27. The race was latent either way.

Fix

• src/vm/backend.rs: avf_rpc connects through a new connect_avf_socket, which retries on ECONNREFUSED with a short backoff (50 → 800 ms, ~1.5 s total), so a genuinely dead runner still fails promptly.
• swift/avf-runner/.../main.swift: the control socket listens with SOMAXCONN instead of 4.

RUNNER_PROTOCOL_VERSION is deliberately not bumped: the client-side retry fixes this against any runner, old or new, so agv doesn't depend on the larger backlog.

Testing

• cargo test passes; cargo clippy is clean; the Swift runner builds.
• Running on the affected VM (macOS 27, AVF, 7 forwards) for several days: no warning on start, and plain ssh <vm> works.
• Slow boot suite not run.
• Note: cargo clippy --all-targets -- -D warnings fails locally on three AVF test files (unfulfilled_lint_expectations for map_unwrap_or), identically on main without this change. It looks like a local clippy 1.94 vs CI 1.99 difference, so CI is the authority there.

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