Fly with the Java port of nether-pathfinder - #19

NetherPathfinderContext asks cheesecake.process.elytra.pathfinder for paths
and lines of sight, with a Chunk object where the native library handed out
a pointer: the chunk packer fills a Chunk, a block update sets a bit on one,
the solver's block lookups cache one, and there is nothing to free. The
Unsafe fill of an all-solid section is Chunk.fillSection. The
dev.babbaj:nether-pathfinder dependency, its maven repository, the nested
jar and the natives it carried for each platform go, and with them the
system check: every system is supported, so NullElytraProcess goes too, and
elytraCustomAllocator is kept only so that a settings file naming it still
loads.

What the native library made necessary is reconsidered, as #18 asked:

- The read-write lock stays, on the same sides as before, so that the
  threads run in the order the flights were tested in. The port needs none
  of it, and dropping it is a follow-up.
- destroy() no longer takes the write lock to free the context. Closing the
  table under a running ray or search only changes what it answers.
- A path calculation that has not answered in thirty seconds rebuilds its
  context instead of leaking one, since there is no hung thread to join.
- UnusableRays stays, for the answers the flight wants: a point is visible
  from itself, a ray that is not finite is not, and an end on a voxel
  corner is decided by the voxel the ray crosses last.

CI checks the jar for the pathfinder's classes instead of the nested jar,
the headless client's proof that the mod ran is a line the provider now
logs, and the detection of the native library's exit(696969) goes. The
README's status bullets say what is left of the three upstream issues.

Closes #18.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

The headless client's proof that the mod ran is the "Cheesecake loaded"
line the provider logs, and the first run of it never matched: the client
colours its log lines, and the escape codes sit between the logger's name
in parentheses and the message, so a fixed string with a space between the
two finds nothing. A pattern that allows anything between them matches the
line as a local client wrote it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Baritone's CachedRegion writes a chunk's two bits per block with
BitSet.toByteArray(): bit n sits in byte n / 8 at position n % 8, the
lowest bit first. The native reader took a byte's bits from the top down
(baritone.cpp, get2Bits: `>> (6 - (i % 8))`) and the port copied it, so
block x of every aligned run of four along x came back as block x ^ 3:
each such run mirrored, in every chunk a search took from the region cache
rather than from the game. Hard to see in flight, since the cache only
fills in what the game has not loaded and terrain in runs of four is close
to its own mirror image, but wrong. The test packed its file the same
wrong way and passed; it now builds the chunk with a BitSet as CachedChunk
does, sets one block solid and one water, and checks that the mirror
positions are air. The region directory is Baritone's `cache`, not
`regions`, in the docs and the test, and the oracle's doc names its real
path. The same fix, on the port's own branch, is 0Mattias/nether-pathfinder
4784d69.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>