Support the HTML5 step attribute on number fields

## Enhancement

Number fields render `` with optional `min` and `max`, but there is no way to set `step`, so users cannot constrain the increment (for example, steps of 5, or decimals).

## Proposal

Add an optional `step` field argument and emit `step="…"` on the input when it is set, cast and escaped like the existing `min`/`max` handling, plus an entry in `custom_metadata_examples.php`.

## History

Originally proposed in #112 by @sonnyt (2014). That PR no longer applies cleanly, so it is being closed in favour of a fresh, clean implementation tracked here.

Start by locating the number-field rendering and its existing min/max handling, then inspect custom_metadata_examples.php for the corresponding example. Add the optional step argument using the same casting and escaping behavior as min/max, emit step when set, and update the example to demonstrate it.

Written by the indexing model from the issue text.