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)

README.md

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+# The Cuckoo Escapement
+
+**What the Raspberry Pi time-server guides get wrong, and the numbers to prove it.**
+
+Docs: **[cuckoo.warehack.ing](https://cuckoo.warehack.ing)**
+
+We built a GPS-disciplined Stratum 1 NTP server on a Raspberry Pi 4, followed the
+published advice, and measured everything. Most of that advice is wrong on this
+board. One piece of it is wrong on *every* board.
+
+| Change | RMS offset |
+|---|---|
+| Baseline | 823 ns |
+| chrony median `filter` + `prefer` on the PPS refclock | 440 ns |
+| PREEMPT_RT, as the guides recommend | **2468 ns** ← *worse* |
+| PREEMPT_RT + our `IRQF_NO_THREAD` patch | **199 ns** |
+
+## The one thing worth stealing
+
+[`kernel/0001-pps-gpio-keep-timestamp-in-hard-irq-under-PREEMPT_RT.patch`](kernel/)
+
+PREEMPT_RT force-threads interrupt handlers. `pps-gpio` takes its timestamp
+*inside* its handler. So the realtime kernel — the marquee upgrade in every guide
+— puts a scheduler between the electrical edge and the clock, and triples your
+jitter. Four lines fix it.
+
+As far as we can tell this isn't applied anywhere, which means anyone running
+GPIO-based PPS on a realtime kernel today is silently eating microseconds of
+jitter with no reason to suspect it. chrony still says Stratum 1. Everything
+still *looks* fine.
+
+## What's here
+
+| | |
+|---|---|
+| `kernel/` | The patch, and how to build it |
+| `dashboard/` | The live status page (FastAPI + WebSocket, no build step) |
+| `docs-site/` | The write-up — Astro / Starlight |
+
+## n = 1
+
+Every number here comes from one Pi 4 and one GPS module. This is a field report,
+not a study. We're publishing the method alongside the results precisely so you
+can check it against your own board rather than take our word for it.
+
+---
+
+A [Supported Systems](https://supported.systems) joint.