Add recorder: raw CAN and GNSS sessions with failsafes and a storage bound

Reads SocketCAN and gpsd directly. Sessions start on the first frame or on
'ctl start', end after 15 s of bus idle or on 'ctl stop'; blips under 3 s are
discarded. candump -L chunks fdatasync'd every second and zstd'd once closed;
oldest sessions deleted to stay under 50 GB / above 20 GB free. Kernel and
queue drops, link changes and clock steps are recorded as events.

Bench-tested on vcan with the first car capture (byte-identical) and installed
on the car Orin as yukart-recorder.service.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

src/recorder/clock.rs

@@ -0,0 +1,40 @@
+//! The car unit has no RTC battery: the clock boots stale and is stepped later by
+//! chrony (NTP or GNSS). Recordings therefore carry enough to correct timestamps
+//! afterwards — sync state at start, monotonic time, and every step observed.
+
+use serde_json::{json, Value};
+
+fn clock(id: libc::clockid_t) -> f64 {
+    let mut ts: libc::timespec = unsafe { std::mem::zeroed() };
+    unsafe { libc::clock_gettime(id, &mut ts) };
+    ts.tv_sec as f64 + ts.tv_nsec as f64 * 1e-9
+}
+
+pub fn realtime() -> f64 {
+    clock(libc::CLOCK_REALTIME)
+}
+
+pub fn monotonic() -> f64 {
+    clock(libc::CLOCK_MONOTONIC)
+}
+
+/// realtime − monotonic. Constant while the clock only slews; a jump means a step.
+pub fn offset() -> f64 {
+    realtime() - monotonic()
+}
+
+pub fn boot_id() -> String {
+    std::fs::read_to_string("/proc/sys/kernel/random/boot_id").map(|s| s.trim().to_string()).unwrap_or_default()
+}
+
+/// Kernel view of synchronisation, read without privilege (adjtimex with modes = 0).
+pub fn sync_state() -> Value {
+    let mut tx: libc::timex = unsafe { std::mem::zeroed() };
+    let state = unsafe { libc::adjtimex(&mut tx) };
+    let unsync = state == libc::TIME_ERROR || tx.status & libc::STA_UNSYNC != 0;
+    json!({ "synced": !unsync, "maxerror_us": tx.maxerror, "esterror_us": tx.esterror })
+}
+
+pub fn hostname() -> String {
+    std::fs::read_to_string("/proc/sys/kernel/hostname").map(|s| s.trim().to_string()).unwrap_or_default()
+}