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init's own DANOS.LOG shutdown flush retire (superseded).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/fs.zig

@@ -247,9 +247,12 @@ pub fn makePath(path: []const u8) bool {
         while (end < path.len and path[end] != '/') end += 1;
         const prefix = path[0..end];
         if (prefix.len == 0 or (prefix.len == 1 and prefix[0] == '/')) continue;
-        if (!exists(prefix) and !makeDirectory(prefix)) return false;
+        // Best-effort per prefix: components at or above a mount point ("/mnt")
+        // are router names, not filesystem nodes — they neither exist as nodes
+        // nor accept mkdir, and that is fine. Only the final verdict counts.
+        if (!exists(prefix)) _ = makeDirectory(prefix);
     }
-    return true;
+    return exists(path);
 }
 
 /// Remove the file at `path`. Returns true on success. Directories are refused