feat: .at_most(n) quantifier closes the symmetry gap with at_least/exactly/between (#265)

Adds `.at_most(n)` on both `RegexBuilder`/`Pattern` and as an
`at_most(n, operand)` factory. Closes the symmetry gap in the quantifier
lineup — we had `exactly(n)`, `at_least(n)`, and `between(lo, hi)`, but
no `{0,n}` shorthand.

## What is new
- `AtMostElement(times, child)` in the elements union, rendered as
`{0,times}` by the quantifier suffix dispatch.
- `.at_most(count)` chain method on `QuantifiersMixin` — validates
`count > 0` (matches the `at_least` contract).
- `at_most(count, operand)` free-function factory in the humre-style
API.
- Re-exported through `edify.pattern` and top-level `edify`.

## Example
```python
from edify import DIGIT, at_most, Pattern

at_most(3, DIGIT).to_regex_string()             # \d{0,3}
Pattern().at_most(3).digit().to_regex_string()  # \d{0,3}
```

Closes #130

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