benchmark: fix calibrate-n option handling

[luanmuniz]

This fixes a few issues in benchmark/calibrate-n.js found while using the tool to calibrate new benchmark values.

Before this change, --cv-threshold and --max-increases were parsed from the wrong substring index. The parsed value still included the = separator, so values such as --cv-threshold=0.035 and --max-increases=1 were read as =0.035 and =1. This made --cv-threshold become NaN and caused --max-increases to fall back to the default value.

This also updates the stability checks and output so the configured threshold is used consistently. Previously, parts of the tool still compared the overall CV against the default CV_THRESHOLD and printed hardcoded 5% / 10% thresholds even when custom values were provided.

Changes included:

• fix parsing for --cv-threshold
• fix parsing for --max-increases
• use the configured cvThreshold when checking overall stability
• validate invalid --runs and --cv-threshold values consistently with the other numeric options
• add named constants for the default starting n value and the per-config CV threshold
• correct the --start-n help text to match the actual already-in-place default value
• update user-facing output so reported thresholds match the values used by the tool

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[RafaelGSS]

LGTM

[luanmuniz]

@RafaelGSS Thanks for the review! I can see the CI is failing due to an issue with the commit message.

I noticed the same problem on my other open PR's, and I already fixed them there by amend+force-push-with-lease.

I reread the guidelines to make sure, and they say that for this type of issue, someone would help get things sorted here. I will await your instructions on the correct flow to fix the CI here.