clock diagnostics for issue #3: probe, VM sanity scenario, fix plan

The qemu/ device sim answered the first question off-board: the same
kernel family under -M virt gives musl vDSO rate ratio 0.99963 — the
generic 6.18 armv7 vDSO is correct, so the board symptom is RV1106
register state (CNTFRQ/CNTVOFF, firmware-owned, secure-world boot chain).

- qemu/tests/clockprobe: interval-based musl probe separating RATE error
  (CNTFRQ) from boot OFFSET (CNTVOFF) — the original single absolute
  sample cannot distinguish them.
- qemu/tests/clock-sanity.sh: VM regression guard asserting the vDSO rate
  within 1% (PASSES: 1.00026); cross-builds the probe and stages it as
  payload itself.
- kernel/rv1106-enablement/timer/PLAN.md: the DT fix
  (arm,cpu-registers-not-fw-configured + measured clock-frequency on the
  board dts) gated on the two bench measurements; c8a3 is currently
  physically dark, needs hands at the bench.
- clockprobe joins the CI test loop.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018HUayid7W5w7jBdb9Rrj1K

qemu/tests/clockprobe/src/main.rs

@@ -0,0 +1,25 @@
+// Differential clock probe for warden-sdk issue #3: compares the musl/vDSO
+// CLOCK_MONOTONIC rate against the kernel's own /proc/uptime over a fixed
+// interval, so boot-time offsets cancel and only the RATE ratio remains.
+use std::{fs, thread, time::Duration};
+
+fn mono(clock: libc::clockid_t) -> f64 {
+    let mut ts = libc::timespec { tv_sec: 0, tv_nsec: 0 };
+    unsafe { libc::clock_gettime(clock, &mut ts) };
+    ts.tv_sec as f64 + ts.tv_nsec as f64 / 1e9
+}
+
+fn uptime() -> f64 {
+    fs::read_to_string("/proc/uptime").unwrap()
+        .split_whitespace().next().unwrap().parse().unwrap()
+}
+
+fn main() {
+    let interval = 10.0;
+    let (m0, r0, u0) = (mono(libc::CLOCK_MONOTONIC), mono(libc::CLOCK_MONOTONIC_RAW), uptime());
+    thread::sleep(Duration::from_secs_f64(interval));
+    let (m1, r1, u1) = (mono(libc::CLOCK_MONOTONIC), mono(libc::CLOCK_MONOTONIC_RAW), uptime());
+    let (dm, dr, du) = (m1 - m0, r1 - r0, u1 - u0);
+    println!("CLOCKPROBE monotonic={dm:.4} raw={dr:.4} uptime={du:.4} ratio_mono={:.5} ratio_raw={:.5}", dm / du, dr / du);
+    println!("CLOCKPROBE abs monotonic={m1:.2} uptime={u1:.2} abs_ratio={:.5}", m1 / u1);
+}