Return practical integer reading times

[Jord-JD]

What changed

• returns rounded-up integer minutes and seconds suitable for display
• adds scalar parameter and return types
• rejects zero or negative reading speeds instead of dividing by zero
• updates tests and adds PHP 7.1/PHP 8.5 CI
• moves the branch alias to 4.0 because the return-type change is intentionally breaking

Why

This resolves #2 and #3 and supersedes #5. A major release is planned because callers may currently depend on fractional values.

Validation

• Composer JSON parsed successfully
• git diff --check passed
• GitHub Actions runs the updated unit suite on the supported floor and PHP 8.5