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/.env.example

@@ -0,0 +1,13 @@
+# Copy to .env. Nothing site-specific belongs in the Makefile.
+PI=deploy@gps-ntp.local
+KEY=~/.ssh/id_ed25519
+APPDIR=/opt/gpsntp-dashboard
+
+# Only needed if you front the dashboard with TLS via `make caddy`.
+DOMAIN=clock.example.internal
+CERT_SRC=$(HOME)/.certs/clock.example.internal
+
+# hidden (default) | coarse (~11 km) | exact
+# The receiver knows exactly where it is. `hidden` keeps that out of the page —
+# and out of any screenshot you post. Think before you change this.
+GPSNTP_POSITION=hidden