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

2 files changed, 11 insertions, 0 deletions

diff --git a/tests/builder/builder.test.py b/tests/builder/builder.test.py
index 062e1ae..33aa17d 100644
--- a/tests/builder/builder.test.py
+++ b/tests/builder/builder.test.py
@@ -348,6 +348,12 @@ def test_at_least():
     regex_compilation("\\w{3,}", expr)
 
 
+def test_at_most():
+    expr = RegexBuilder().at_most(3).word()
+    regex_equality("\\w{0,3}", expr)
+    regex_compilation("\\w{0,3}", expr)
+
+
 def test_between():
     expr = RegexBuilder().between(3, 5).word()
     regex_equality("\\w{3,5}", expr)
diff --git a/tests/builder/validation.test.py b/tests/builder/validation.test.py
index e4898ad..53128c5 100644
--- a/tests/builder/validation.test.py
+++ b/tests/builder/validation.test.py
@@ -109,6 +109,11 @@ def test_at_least_non_positive_raises():
         RegexBuilder().at_least(-1).digit()
 
 
+def test_at_most_non_positive_raises():
+    with pytest.raises(MustBePositiveIntegerError):
+        RegexBuilder().at_most(0).digit()
+
+
 def test_between_negative_lower_raises():
     with pytest.raises(MustBeIntegerGreaterThanZeroError):
         RegexBuilder().between(-1, 5).digit()