logger: per-process log files under /var/log/<boot-stamp>/

The logger service drains the tagged ring every 250 ms and demultiplexes
it into one file per process on the flash volume:

    /mnt/usb/var/log/2026-07-21T150434Z/system/services/fat.log
    [     0.214] mounted FAT (fat32, 128992 clusters, partition lba 0)

The boot stamp is the wall-clock anchor from klog_status (FAT-safe, no
colons); the kernel's records go to kernel.log; each line carries the
record's monotonic timestamp and level. Storage is best-effort and
late — the ring buffers a whole boot until the mount appears, then the
first drain writes the backlog, storage-stack records included. Lost
records surface as '-- N records lost --' from sequence gaps. Files
close (= fat's device cache flush) after a 2 s quiet period, bounding
data-at-risk without per-record flush thrash. The logger announces
itself once — a periodic status line would feed the stream it drains.

init spawns the logger last, so the reverse-order shutdown stops it
first and its final drain runs over a live storage chain; log-flush and
init's own DANOS.LOG shutdown flush retire (superseded). runtime.fs
makePath treats components at or above a mount point as router names —
create is best-effort per prefix, the final verdict is exists(path).

library/runtime/system.zig

@@ -26,6 +26,12 @@ pub fn yield() void {
 pub const KlogLevel = abi.KlogLevel;
 pub const KlogStatus = abi.KlogStatus;
 pub const KlogRecordHeader = abi.KlogRecordHeader;
+pub const klog_record_header_size = abi.klog_record_header_size;
+pub const klog_record_alignment = abi.klog_record_alignment;
+pub const klog_record_magic = abi.klog_record_magic;
+pub const klog_flag_truncated = abi.klog_flag_truncated;
+pub const klog_maximum_message = abi.klog_maximum_message;
+pub const maximum_process_name = abi.maximum_process_name;
 
 /// Write raw bytes to the kernel log (bring-up/panic diagnostics; ordinary
 /// output goes through std.log -> writeRecord). The kernel stamps the record