v0.2.5 - stratum & dispatch correctness fixes (issue #2)

Six correctness fixes in the stratum and dispatch paths, all from an external
audit (issue #2, thanks @odiseusme) and verified against the code:

1. Robust socket writes - the send path now writes the whole buffer or fails,
   so a short write can't send truncated JSON as a successful share; it uses
   MSG_NOSIGNAL and ignores SIGPIPE, so a pool disconnecting mid-send no longer
   kills the miner.
2. Job id travels with the job - a new block arriving mid-batch can no longer
   cause a solution to be submitted under the wrong job id and rejected.
3. Nonce stays in your subspace - a launch that would cross the owned-nonce
   boundary is clamped, so no work leaks into the pool's extranonce prefix.
4. --batch is validated - clamped to a non-zero multiple of 256, closing the
   overlap / infinite-retest / skip-and-overstate cases.
5. Vulkan memory barrier - the dataset build's writes are now a guaranteed
   dependency for the search reads, not just fence-ordered.
6. Lenient JSON - trailing whitespace in a pool's numeric fields no longer
   silently drops the subscribe reply onto the default extranonce.

No dev fee. make test green on both backends.