fix(daemon): bind-before-heavy-init single-flight for concurrent spawns (#1010)

[zackees]

Fixes #1010. N concurrent fbuild invocations with no daemon running (common CI / parallel-build pattern) all spawned a full fbuild-daemon, each paying for the embedded zccache service + DaemonContext + package managers before the LATE port bind finally failed — N heavyweight processes to leave one.

1. Daemon: bind before heavy init. Moved bind_listener_with_retry to the top of main (after tracing setup), ahead of the zccache service / DaemonContext / app build. A daemon that loses the bind race detects it in milliseconds and exits — no wasted init.
2. Fast, correct collision handling. The endpoint is now version+identity keyed (daemon: endpoint isolation follow-up to #940 — key the default endpoint by cache-identity/checkout + deterministic version arbitration #1009), so anything listening on the port is a daemon of our own version. New another_daemon_is_listening() distinguishes "a live daemon owns it → yield (exit 0)" from "stale TIME_WAIT → retry". Losers exit 0 cleanly instead of retrying 1.5 s then exiting 1.
3. CLI: adopt-before-spawn. ensure_direct_daemon_running re-checks health before each spawn so a concurrent invocation's just-started daemon is adopted, not piled onto.

No file locks (banned by ban_file_based_locks) — the OS-exclusive port bind is the election; the daemon makes losing it cheap.

Verified — local + Linux Docker

cargo check --workspace green (14.5s); another_daemon_is_listening test (live listener → yield, free port → retry) passes on Linux Docker. clippy -D warnings + fmt clean.

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Reviewing files that changed from the base of the PR and between 0bb7252 and dab2dff.

📒 Files selected for processing (2)

• crates/fbuild-cli/src/daemon_client.rs
• crates/fbuild-daemon/src/main.rs

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• Create PR with unit tests
• Commit unit tests in branch fix/1010-daemon-spawn-single-flight

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