Guard benchmark summary against zero-time compare ratios

[Copilot]

This addresses the remaining benchmark.sh review-thread issue in PR #217. The compare summary could divide by zero when the branch average rounded to 0, breaking output for very small runs.

• Summary ratio handling

  • Guard cmp/branch computation when branch_avg == 0
  • Emit stable summary values instead of dividing by zero:
    • inf when compare is non-zero and branch rounds to zero
    • n/a when both sides round to zero

• Summary labeling

  • Make the zero-rounded case explicit in the note column with branch ~0ms

• Scope

  • Limit changes to the summary awk block in benchmark.sh
  • Leave normal non-zero timing behavior unchanged

# before
cmp_branch = u2 / u1

# after
if (u1 > 0) {
  cmp_branch = u2 / u1
  sp = sprintf("%10.2fx", cmp_branch)
} else if (u2 > 0) {
  cmp_branch = "inf"
  sp = sprintf("%10s", "inf")
} else {
  cmp_branch = ""
  sp = sprintf("%10s", "n/a")
}