opencodecs 0.3.1

A CZI read scheduling fix, and a clear refusal for the files that cannot stack.
Figures compare against 0.3.0 on the same machines.

CZI whole-stack reads hand out work by bytes

read derived its batch size from the worker count, giving each worker
ceil(n/workers) consecutive sub-blocks. That makes the read wait on that many
sub-blocks even when most workers have already finished: nine 8 MiB sub-blocks
over eight workers is two rounds, and measured 15.1 ms against 7.9 ms for nine
one-sub-block tasks. Tasks are now sized by output bytes, about 8 MiB each, so a
large sub-block gets a task to itself and small ones ride together. That keeps
the reason batching existed, which is that a 0.7 MiB tile cannot pay for its own
future, without paying the rounding.

_READ_MAX_WORKERS goes 8 to 32: the old value was measured against the
dispatch above, where extra workers could not help. With byte-sized tasks, 20 to
32 measured fastest on both a 20-core arm64 Mac and a 128-core x86-64 Linux
host, and past 32 the Linux host gives it back to memory-bandwidth contention. A
semaphore keeps the resolved worker count a real bound when the byte sizing
yields more tasks than that.

Eight files of 2000 x 2000 uint16 ZSTDHDR sub-blocks, read back to back: 22.1 to
about 15 ms per file on macOS, which moves a whole-stack read from a little
behind aicspylibczi to about 1.3x ahead of it.

A mixed sub-block CZI says so, instead of failing inside the codec

read stacks every sub-block into one array, and sized that array from the first
sub-block, which is wrong for any file whose sub-blocks differ: a pyramidal slide
scan interleaves down-scaled sub-blocks with full-resolution ones, and a mosaic
can clip its edge tiles. The decode destination check then failed with an element
count the caller never chose, naming neither the file's shape mix nor a way
forward. It now refuses up front, listing the shapes it cannot stack and the APIs
that do work (CziPyramidReader.read_region, entries_at_level, read_tile and
iter_tiles), and the new CziReader.is_uniform lets a caller branch before
asking. Decoding was never at fault: JPEG XR sub-blocks are pixel-exact against
aicspylibczi.