fix: post-merge correctness + hang fixes

[BlueBirdBack]

Four small fixes found in post-merge review:

• abs.sh:1849 — 'Disk random write 4K QD1' displayed read-side p95 (wrote 'p95 FAILED ms' on pure-write jobs). Use write_p95_ms.
• run_iperf_cmd — watchdog was hardcoded to 18s, so IPERF_TIME>~13s was always killed mid-test and reported FAILED. Budget now follows IPERF_TIME + slack, capped at ABS_CMD_TIMEOUT.
• abs_timeout — single SIGTERM could hang the whole script forever if the child ignored TERM (e.g. uninterruptible I/O). Escalates to SIGKILL after a 3s grace.
• dd_fallback — wrote a fixed 256 MiB file with no space check; on a nearly-full fs this could fill the disk. Skips when <384 MiB free.