CUBELinux.0.3: the kernel writes, and is compared while writing

0.2 read the store. This appends to it: a mutation through /dev/cubelinux is
written to the log and fsynced *before* the write is accepted, which is the
kernel's version of the contract cube_duratest.py proves for the daemon.

- the mutation comes in as the argument block the coordinate interface will
  pass: op(1) | space(32) | x(8) | y(8) | z(8) | len(4) | value[len]. The kernel
  Morton-encodes the point itself — a key written here has to be the key a
  userspace reader decodes, and that is a thing the gate would catch if it were
  merely similar;
- the append lands after the log's valid prefix, so a torn tail is overwritten
  rather than appended to, exactly as the userspace log does it;
- a log region that has never been written is zeros, not a log: the header is
  written first, the way the userspace log creates its file;
- the entry's CRC covers space, key, length and value, computed the same way.

The write is the byte plane — bytes at a coordinate, no header written beside
them — because that is what a differential comparison against userspace's
`cell put` can be exact about. The header tier sits above this.

Gate (kernel/verify-kernel-append.sh), four mutations including an empty value
and a record in a second space:

  reference : bytes=500 records=6 value_bytes=94 fnv1a64=6b679d39597a62b3
  kernel    : bytes=500 records=6 value_bytes=94 fnv1a64=6b679d39597a62b3
  folded    : bytes=500 records=6 value_bytes=94 fnv1a64=6b679d39597a62b3
  survived  : bytes=500 records=6 value_bytes=94 fnv1a64=6b679d39597a62b3

The third line is the one that matters: userspace *reads the log the kernel
wrote*, folds it, and lands on the same store. Without it the first two would
only show the kernel agreeing with itself. The fourth is a SIGKILL of the VM
with no shutdown and therefore no flush for us.

Not yet: the kernel folds nothing itself, so it still depends on a userspace
checkpoint to reclaim its log. That is the next step, and it has its own gate.

Makefile

@@ -9,7 +9,7 @@ NAME = CUBELinux
 # release. The base version stays in VERSION/PATCHLEVEL/SUBLEVEL above (visible in
 # `make kernelversion`), while `uname -r` and /lib/modules report the CUBELinux
 # release, with any -dirty or SCM suffix still appended by setlocalversion.
-CUBELINUX_VERSION = CUBELinux.0.2
+CUBELINUX_VERSION = CUBELinux.0.3
 
 # *DOCUMENTATION*
 # To see a list of typical targets execute "make help"