Reject NaN timeout durations before scheduling

[OskarEichler]

Summary

Reject NaN before creating a timeout request or delegating to a fiber scheduler. Test for a non-negative comparison instead of only testing for a negative comparison; NaN satisfies neither.

Reproduction

require 'timeout'
ran = false
begin
  Timeout.timeout(Float::NAN) { ran = true }
rescue => error
  p error.class # after: ArgumentError
end
p ran # before: true; after: false

With an active block, the old request has a NaN deadline: neither its wait condition nor its expiry comparison succeeds, so the watcher repeatedly revisits it. An isolated bounded reproduction observed the watcher running until the request was cancelled. No throughput/CPU benchmark claim is made.

Focused checks cover both exception modes, nil/zero/positive integer/float/rational values, negative/infinite-negative values and rejection before scheduler delegation.

Verification

• Ruby 4.0.6 via rbenv; reviewed master 295667aac44220ddfd10db014e766ae8d313cfb8.
• Existing bundle exec rake test: 36 tests / 81 assertions / zero failures or errors, baseline and this branch. Includes the existing signal, fork, Ractor, exception and mocked-scheduler cases.
• nan: 15 checks, 0 failures in a disposable external script, with the affected expectations reproduced before the change.
• Syntax and git diff --check pass. Supplemental RuboCop Lint retains the same two existing offenses; no lint-clean claim.
• No repository tests added/modified, following the consuming project's explicit no-new-tests instruction. Standalone reproduction is included above.
• Existing issues/PRs checked, including multi threaded use of timeout leads to OOM on high throughput scenarios #58 throughput, Handle nested timeout calls with the same duration #91 nested-timeout draft, Ensure thread is released on completion #114 completed-thread retention, and historic comparison/deferred-exception changes. No duplicate retention or nested-timeout proposal.

Compatibility and limitations

NaN now raises ArgumentError without yielding, matching the documented non-negative-number requirement. Negative inputs retain that error. This does not restore the Numeric class restriction removed in 7d2af46; existing duck typing and zero/nil behavior remain. Positive infinity and very large intervals are a separate issue, not fixed in this PR.

The same runtime file is shipped in 0.6.1; this patch can be adopted without a release upgrade. Ruby >= 2.6 and the empty runtime dependency set remain unchanged. No production access or external network diagnostic was used; other Ruby/OS combinations and full scheduler implementations were not run locally.