fix(bench): warn on all reset_vm_hwm errors, not just PermissionDenied

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

crates/bench/src/metrics.rs

@@ -80,14 +80,6 @@ impl MetricsCollector {
         })
     }
 
-    /// Check if we can reset VmHWM (i.e., can open `/proc/self/clear_refs` for writing)
-    pub fn can_reset_vm_hwm() -> bool {
-        std::fs::OpenOptions::new()
-            .write(true)
-            .open("/proc/self/clear_refs")
-            .is_ok()
-    }
-
     /// Get file inode number
     pub fn get_inode(path: &Path) -> std::io::Result<u64> {
         let meta = fs::metadata(path)?;