Issue #38: warn_reset_hwm() silently swallowed non-permission I/O errors
from /proc/self/clear_refs (e.g. missing /proc, read-only procfs, kernel
incompatibility). This left users unaware that VmHWM reset failed and
memory peak data could be contaminated across suites.
Changes:
- runner.rs: all errors now produce a warning with specific failure reason;
PermissionDenied retains 'try running as root' hint; AtomicBool warn-once
prevents duplicate output across 7 suite runs
- main.rs: preflight check now uses warn_reset_hwm() instead of the vague
can_reset_vm_hwm(), sharing the same warn-once mechanism
- metrics.rs: remove dead can_reset_vm_hwm() (no callers remaining)
- tests: add hwm_warned_flag_prevents_reentry and warn_reset_hwm_does_not_panic
Replace `static mut OLD_SIGBUS_HANDLER` with AtomicU8 + AtomicPtr to
remove data race UB when concurrent benchmarks call open() from multiple
threads.
Key changes:
- Use `Once::call_once` to guarantee single handler installation
- Publish old handler to atomics BEFORE installing new handler (closes
the handler-active-but-state-unpublished race window)
- Read atomics with Acquire in signal handler (async-signal-safe)
- Align si_addr to page boundary before mmap(MAP_FIXED)
- Add concurrent test: 8 threads open all 5 variants simultaneously
