The Cuckoo Escapement: field report, kernel patch, dashboard

What the Raspberry Pi time-server guides get wrong on a Pi 4, with the
measurements. The headline artifact is a four-line pps-gpio patch: PREEMPT_RT
force-threads IRQ handlers, and pps-gpio takes its timestamp inside its handler,
so the realtime kernel puts a scheduler between the electrical edge and the
clock. IRQF_NO_THREAD takes RMS offset from 2468 ns to 199 ns.

- kernel/     the patch
- dashboard/  live status page (position hidden by default)
- docs-site/  the write-up (Astro/Starlight, brass, no tutorial section)

dashboard/deploy/caddy-affinity.conf

@@ -0,0 +1,10 @@
+# systemd drop-in: /etc/systemd/system/caddy.service.d/affinity.conf
+#
+# Keep Caddy off cpu0. The PPS interrupt is handled there and cannot be moved
+# (Pi 4 GPIO IRQs are demuxed via pinctrl-bcm2835 and refuse an smp_affinity),
+# so any work scheduled on cpu0 adds jitter to the PPS timestamp directly.
+# Caddy is mostly idle, but on this box cpu0 belongs to the clock. See
+# TIMING-NOTES.md.
+[Service]
+CPUAffinity=1
+Nice=10