fix(serve): sample play_start_us from CLOCK_MONOTONIC_RAW

[kiwipaulrob]

Problem

In sendspin/serve/coordinator.py, play_start_us is sampled with time.monotonic() -- but the aiosendspin server clock is CLOCK_MONOTONIC_RAW on Linux (aiosendspin/clock.py: not slewed by NTP/adjtime). play_start_us is an absolute timestamp consumed verbatim by workers/clients, so sampling it from a different clock domain introduces a fixed offset equal to every NTP adjustment since boot.

Evidence

On my Linux host the two clocks are currently 102.2 s apart, so the multi-worker serve path timestamps audio ~102 s off. One honest caveat: aiosendspin/clock.py notes the rate mismatch is capped at ~500 ppm and below backpressure tolerances -- but that covers relative durations, not an absolute epoch offset, which is what play_start_us is.

Fix

Adds a small server_now_us() helper (raw clock on Linux, monotonic_ns fallback elsewhere) and uses it for the two absolute-timestamp samples. Interval-only measurements (stats/health timers) deliberately stay on time.monotonic().

Verification

• pytest tests/serve/test_coordinator.py: 7 passed; ruff check clean
• Live check: helper returns the raw-domain value

Happy to adjust the approach if you prefer reusing an aiosendspin clock helper.

[kiwipaulrob]

Hi @OnFreund — these four are now ready for review (were drafts while I finished testing). Small, separate fixes:

• fix(serve): sample play_start_us from CLOCK_MONOTONIC_RAW #282 serve: sample play_start_us from CLOCK_MONOTONIC_RAW (102 s skew reproduced live on this host)
• fix(utils): log fire-and-forget background task failures #283 utils: log fire-and-forget background task failures
• fix(daemon): keep reconnecting after unexpected errors instead of exiting 0 #284 daemon: keep reconnecting after unexpected errors instead of exiting 0
• fix(settings): write settings file atomically via temp + rename #285 settings: atomic write via temp + rename

One ask I can't do myself: the CI runs on all four are frozen at action_required (first-time contributor gate), so there are no checks to show yet. A one-time workflow approval would unblock them. Happy to adjust anything on review.

[OnFreund]

Thanks @kiwipaulrob. I approved and merged this one. The others either fail CI or have a review comment.