fix!: quantifier validation — dangling and stacked quantifiers raise; Hypothesis property gate (#269)

Tightens quantifier semantics so the builder can no longer silently drop
a quantifier from the emitted regex, and locks the invariant in with a
Hypothesis property test.

## Dangling quantifier now raises
`RegexBuilder().exactly(3).to_regex_string()` used to emit `^$` — the
quantifier had no operand, so it was silently discarded. It now raises
`DanglingQuantifierError` at emit time.

```python
RegexBuilder().exactly(3).to_regex_string()
# edify.errors.quantifier.DanglingQuantifierError:
# Dangling quantifier with no operand. Append an element (e.g. .digit()) before compiling.
```

## Stacked quantifiers now raise
`RegexBuilder().one_or_more().exactly(3).digit()` used to emit `\d{3}` —
the second quantifier overwrote the first, silently dropping
`one_or_more`. It now raises `StackedQuantifierError` at chain-call
time.

```python
RegexBuilder().one_or_more().exactly(3).digit()
# edify.errors.quantifier.StackedQuantifierError:
# Cannot stack a quantifier on top of another pending quantifier.
# Add an operand between the two quantifiers or drop one.
```

## Property assertion
Hypothesis-driven test in `tests/builder/properties.test.py` — for any
list of `(quantifier method, element method)` pairs, the emitted regex
is exactly the concatenation of `<element><suffix>` fragments in order.
`hypothesis>=6.100` added to the `dev` dependency group.

## Breaking
Any chain that previously silently emitted a wrong-but-parseable regex
now raises. Correct chains are unaffected.

Closes #109
Closes #110
Closes #111

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