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)

docs-site/.dockerignore

@@ -0,0 +1,19 @@
+node_modules/
+dist/
+.astro/
+
+.env
+.env.local
+.env.production
+
+.git/
+.gitignore
+
+*.log
+.DS_Store
+.vscode/
+.idea/
+
+# don't pull in repo-root artifacts
+../artifacts/
+README.md