fix(cli): reject --fail-under outside 0 to 100 with exit 2 (#1037)

[minutechreview]

Summary

faircode profile --fail-under N accepted any number. The score is 0 to 100, so --fail-under 150 failed every run and --fail-under -5 could never fail, which turns a CI gate off without a warning.

Now a value outside 0 to 100 (also nan and inf) exits 2 with:

error: --fail-under must be between 0 and 100, got 150

The check runs at the start of profile, next to the existing --proxy-alpha range check, before the dataset is read. 0 and 100 stay valid.

• faircode/cli.py: the range check
• faircode/SPEC.md: a paragraph in section 5 about the gate and its range
• README.md: one sentence in the CI paragraph
• tests/test_cli.py: six bad values exit 2 with the message and print nothing to stdout; 0 and 100 still work

Type

• Audit
• Explainer
• Bug fix
• Other

---

Linked issue

Closes #1037

Checks: pytest tests --ignore=tests/browser gives 818 passed, 21 skipped. ruff check faircode tests scripts, scripts/check_em_dash.py and scripts/check_broken_links.py pass.

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@yakew7 @Shreyash0712 @ahmdkaml - new PR touching a path you own, please review.