[kahrendt]

Summary

On Linux, now_us() read CLOCK_MONOTONIC, whose rate NTP slews (by
up to 500 ppm). Every timestamp the server sends follows that rate, so
the clients' time filters had to track each change the NTP daemon made.
The server clock now runs at the hardware's own rate.

Change

• now_us() reads CLOCK_MONOTONIC_RAW on Linux. ESP-IDF keeps its
  esp_timer CLOCK_MONOTONIC, and macOS keeps mach_continuous_time,
  neither of which NTP slews.
• Docs: clock.h and SendspinServer::now_us() drop the claim that
  the clock counts through system sleep on Linux, which neither clock
  did.
• The host's Timer re-arms a wait that ends before now_us() reaches
  its deadline. Asio waits on the slewed CLOCK_MONOTONIC, which can run
  ahead of the raw clock, and a caller that rechecks and returns would
  never run again: the drain check would leave on_drained pending, and
  SessionDriver skips re-arming an unchanged deadline. ESP-IDF's loop
  already fires only once now_us() reaches the deadline.

Tests

• Clock.ReadsTheMonotonicClockInMicroseconds reads
  CLOCK_MONOTONIC_RAW as its reference on both hosts.
• No test for the early wake: a test cannot make the steady clock run
  ahead of the raw one. The Linux path was compiled with -Werror and run in a
  container, where now_us() matched CLOCK_MONOTONIC_RAW.
• Gap: on x86_64 kernels older than 5.3, CLOCK_MONOTONIC_RAW is a
  syscall rather than a vDSO read.

[Copilot]

🟡 Changes recommended

The Linux clock test needs an updated reference, and early timer expiry can leave drain completion pending indefinitely.

2 open issues

• Rearm drain checks when the timer fires before the deadline · New
• Use CLOCK_MONOTONIC_RAW in the Linux clock test · New

What changed in this PR

This PR switches the server’s Linux timestamp clock to CLOCK_MONOTONIC_RAW so NTP adjustments no longer change its rate.

Changes:

• Switch Linux clock reads while preserving macOS and ESP-IDF behavior.
• Correct clock documentation and clarify steady-clock timer drift.

File	Description
src/​transport.h	Clarifies timer clock drift.
src/​clock.h	Documents platform clocks and NTP independence.
src/​clock.cpp	Selects CLOCK_MONOTONIC_RAW on Linux.
include/​sendspin/​server/​server.h	Updates the public clock description.

_{Review effort: Balanced}

[Copilot]

🟢 Approval recommended

Static review found no blocking issues in clock selection or timer lifecycle handling; tests were not run.

No open issues 🎉

2 resolved since last review

• Use CLOCK_MONOTONIC_RAW in the Linux clock test
• Rearm drain checks when the timer fires before the deadline

_{Review effort: Balanced}