Investigate pysam pipe close error during bounded remote BAM extraction #106

During Isovar's catalogue-wide SV RNA audit, bounded remote BAM extraction emits an unraisable pysam close error while handling a record-limit exception:

osteosarc.errors.RecordLimitError: Acquisition exceeds record limit
During handling of the above exception, another exception occurred:
OSError: [Errno 29] Closing failed: Illegal seek
Exception ignored in: 'pysam.libcalignmentfile.AlignmentFile.__dealloc__'
  File "osteosarc/reads.py", line 251, in _run_bounded
    with pysam.AlignmentFile(process.stdout, "rb") as bam:
OSError: [Errno 29] Closing failed: Illegal seek

Observed with released osteosarc 0.15.1, Python 3.12 on macOS, indexed remote RNA BAMs, max_records=500000, and nested batch retries after a record cap. The record-cap handling continues, but the close diagnostic is noisy and its interaction with timer-driven process termination should be checked. A separate leaf acquisition returned CalledProcessError after a truncated BGZF read; the current Isovar receipt records the error class/command but does not preserve the subprocess stderr, so these observations should not be assumed to have the same cause.

Public source in the affected pilot:
https://sid-sijbrandij-osteosarc-dataset.s3.us-west-2.amazonaws.com/kamil/tumor/output/T2/outs/per_sample_outs/T2/sample_alignments.bam

Related implementation and reproducible frozen region plan: openvax/isovar#433 (batch e11a46eac1e8c5e207fcf974, recursively partitioned). Add a pipe/timeout/early-record-cap regression that verifies the primary exception survives cleanup and no partial BAM is published.