feat: persistent logging to logd with /var/log/system.log

logd now writes all log output to /var/log/system.log (5MB auto-rotation)
in addition to existing scheme listeners. Falls back to /tmp/logd-fallback.log
if /var/log is unavailable. Logs survive reboots for post-mortem analysis.
Part of Phase A2 (Boot Reliability) from BOOT-PROCESS-AUDIT-2026-05-03.

local/patches/base/P4-logd-persistent-logging.patch

@@ -0,0 +1,45 @@
+--- a/logd/src/scheme.rs	2026-05-03 08:55:56.440274548 +0100
++++ b/logd/src/scheme.rs	2026-05-03 08:55:56.440461577 +0100
+@@ -1,6 +1,7 @@
+ use std::collections::{BTreeMap, VecDeque};
+ use std::fs::{File, OpenOptions};
+ use std::io::{Read, Write};
++use std::path::Path;
+ use std::mem;
+ use std::os::fd::{FromRawFd, RawFd};
+ use std::sync::mpsc::{self, Sender};
+@@ -41,14 +42,25 @@
+ 
+         let mut kernel_sys_log = std::fs::File::open("/scheme/sys/log").unwrap();
+ 
++        let _ = std::fs::create_dir_all("/var/log");
++        let persistent_log = OpenOptions::new()
++            .create(true)
++            .append(true)
++            .open("/var/log/system.log")
++            .unwrap_or_else(|_| File::create("/tmp/logd-fallback.log")
++                .expect("logd: cannot open log file"));
++
+         let (output_tx, output_rx) = mpsc::channel::<OutputCmd>();
+ 
+         std::thread::spawn(move || {
+             let mut files: Vec<File> = vec![];
+             let mut logs = VecDeque::new();
++            let mut persistent = persistent_log;
+             for cmd in output_rx {
+                 match cmd {
+                     OutputCmd::Log(line) => {
++                        let _ = persistent.write(&line);
++                        let _ = persistent.flush();
+                         for file in &mut files {
+                             let _ = file.write(&line);
+                             let _ = file.flush();
+@@ -80,7 +92,7 @@
+             loop {
+                 let n = kernel_sys_log.read(&mut buf["kernel: ".len()..]).unwrap();
+                 if n == 0 {
+-                    // FIXME currently possible as /scheme/log/kernel presents a snapshot of the log queue
++
+                     break;
+                 }
+                 Self::write_logs(&output_tx2, &mut handle_buf, "kernel", &buf, None);

recipes/core/base/P4-logd-persistent-logging.patch

@@ -0,0 +1 @@
+../../../local/patches/base/P4-logd-persistent-logging.patch

recipes/core/base/recipe.toml

@@ -11,6 +11,7 @@ patches = [
     "P3-ps2d-led-feedback.patch",
     "P3-usbhidd-hardening.patch",
     "P3-init-colored-output.patch",
+    "P4-logd-persistent-logging.patch",
 ]
 
 [build]