fs: devtmpfs, so /dev is generated rather than written to a disk

A fresh boot spent 2600 ms in schinit's bootstrap() before printing its first
line, and the whole of it was device nodes. Measured rather than guessed: an
instrumented bootstrap reported 2600 ms on a fresh image and 0 ms on the next
boot, because the nodes already existed by then and mknod failed fast. The cost
was therefore invisible to anyone not building images -- which is to say,
invisible to everyone except whoever works on this.

Each node was a real inode on the root filesystem, and SchFS commits every
mutation as its own transaction group: a copy-on-write rewrite of the path to
the root, then an uberblock behind a cache flush. Twenty of those is seconds.

The duplication mattered more than the time. Major and minor numbers are the
kernel's -- devsw[] in kernel/dev/cdev.c is what they dispatch on -- and schinit
held a second copy of that mapping with nothing checking the two agreed. Adding
a character device meant editing a userspace program, and a wrong minor produced
a node that opened successfully and did nothing.

kernel/fs/devfs.c puts the table next to the thing it describes and serves /dev
behind the same fs_ops vtable procfs uses. The namespace is all it provides:
fileread and filewrite see T_DEV and go through devsw themselves, so a read
never reaches it. It is read-only, and dirlink and ialloc answer EROFS rather
than pretending -- a node exists because the kernel has the device, or it does
not exist.

This also makes /proc/filesystems true. It has advertised "nodev devtmpfs" for
as long as it has existed, against a kernel that had no such filesystem.

Two things found while measuring what remained:

  * The thirteen mknod() calls for /proc ran *after* the procfs mount, so every
    one was resolving a path into a read-only synthetic filesystem and failing.
    procfs.c's own note called them a fallback to be cleaned up later; they have
    been dead for longer than that.
  * The four mkdir() calls for mount points were the rest of it. They are made
    by the image builder now (sysroot-install), and the ones in init are a
    fallback -- a mkdir that fails because the directory exists costs a path
    walk and no writes, where creating one costs a transaction.

bootstrap() is now 0 ms on a fresh image, down from 2600.

init opens /dev/console after the mount rather than mknod'ing a node called
"console" into the root directory, which is what used to leave a stray /console
in `ls /`. The old path is kept as a fallback: if the mount fails there is no
/dev/console and no way to report it.

socktest passes, /proc still lists per-process directories, ps -e still
enumerates.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

kernel/fs/fs.c

@@ -279,6 +279,7 @@ static const struct fs_ops v1_ops = {
 extern const struct fs_ops schfs_ops;
 // Defined in fs/procfs.c -- the synthetic /proc.
 extern const struct fs_ops procfs_ops;
+extern const struct fs_ops devfs_ops;
 
 // Is this filesystem read-only?
 //
@@ -300,6 +301,7 @@ ops_for(uint dev)
   // kmount.c turns that device into the right volume.
   if (SCHFS_IS_ANY_DEV(dev)) return &schfs_ops;
   if (dev == PROCFS_DEV) return &procfs_ops;
+  if (dev == DEVFS_DEV)  return &devfs_ops;
   return &v1_ops;
 }