Feat: Add step property for number rule

[E1ecti]

Implementing step property maintains consistency with the HTML5 range and number input types, which utilize the step attribute.

[icebob-ai]

Thanks for the well-structured PR — all the pieces are there (rule, messages, types, tests, README). Nice work!

However, there's a floating point precision bug with the modulo approach:

0.3 % 0.1  // → 0.09999999999999998 (not 0!)
1.2 % 0.4  // → 0.3999999999999999

So { type: "number", step: 0.1 } would incorrectly reject 0.3. The current tests only use integer steps (step: 10), which hides this issue.

Please fix:

1. Handle floating point precision — e.g. using a tolerance check like Math.abs(Math.round(value / step) - value / step) > 1e-10 or a similar approach
2. Add test cases with float steps (e.g. step: 0.1 with values like 0.3, 0.7, 1.2)
3. Consider validating that step is a positive number (what should step: -5 or step: 0 do?)

Small effort, but it'll make this feature solid. Thanks!

[E1ecti]

Thanks for the feedback!

I've implemented the suggested tolerance check to handle floating-point precision issues. Regarding the step schema validation: while negative values don't technically break the logic, I've added a check to ensure it's a positive number.

I also updated the test suite to cover floating-point scenarios and invalid schema configurations.

[E1ecti]

Previous tolerance-based approach allowed certain invalid values to pass, which was incorrect behavior for the validator.

Current implementation eliminates false positives for floating-point numbers exceeding the old 1e-10 precision limit (such as 1.00000000001).

[icebob-ai]

Looks great now! The floating point handling with the integer multiplication approach is solid, schema validation catches invalid steps at compile time, and the test coverage is thorough (including float edge cases).

Minor nit: there are a few unrelated whitespace changes (double-space → single-space in makeError calls) — ideally those would be in a separate commit, but it's not a blocker.

Thanks for the quick turnaround and the quality work — merging! 🎉