fix: validate memory measurement is numeric before display

[BryanFRD]

Summary

• Fix measure_memory to check /usr/bin/time exists before calling it
• Validate extracted RSS value is numeric before formatting
• In markdown output, only accept .mem values matching ^[0-9]+\.?[0-9]*$
• Prevents 46.6N/A MB display when memory measurement partially fails in Docker

Test plan

• Run benchmarks in Docker environment
• Shellcheck clean

[Copilot]

Pull request overview

This PR hardens peak memory (RSS) reporting in the benchmark runner to avoid rendering malformed memory strings when measurement fails.

Changes:

• Add a Linux + /usr/bin/time availability guard before attempting RSS measurement.
• Validate that the extracted RSS value is numeric before converting/formatting it.
• In markdown output, only accept .mem values that match a numeric pattern before displaying them.

[Copilot]

With set -euo pipefail, the raw=$(...) pipeline can terminate the entire script if any stage returns non-zero (e.g., /usr/bin/time -v not supported, or the expected "Maximum resident" line isn’t present and grep exits 1). Since this function is intended to fall back to N/A, make the extraction pipeline non-fatal (e.g., append || true inside the command substitution, or avoid grep by using a single awk that yields empty output and exits 0 when no match is found).