feat: builder ergonomics — __repr__, __eq__/__hash__, .fork()/.copy() (#268)

Three builder-ergonomics wins so `RegexBuilder` / `Pattern` behave like
proper Python values in interactive shells, dicts, and comparisons.

## `__repr__` shows the pattern-so-far
Replaces the useless default `<edify.builder.builder.RegexBuilder object
at 0x...>` with `<RegexBuilder '^\d+'>`. Builders whose frames are still
open render as `<RegexBuilder '<unclosed>'>` so the repr never raises.

## `__eq__` / `__hash__` — value equality
Two builders are equal when their underlying immutable `BuilderState`
matches; the hash derives from the same state. Builders become safe as
dict keys, set members, or assertion targets. `RegexBuilder` and
`Pattern` share every mixin below the class name, so a builder and
pattern with identical state compare equal.

**Breaking:** identity-based `==` on builders becomes value-based. Only
code that intentionally relied on identity equality is affected.

## `.fork()` / `.copy()` — explicit branch construction
Chaining already yields implicit forking (immutable state means
`root.digit()` and `root.word()` share nothing after the split);
`.fork()` and `.copy()` are aliases that surface the property via
autocomplete and signal branching intent in code.

## Example
```python
from edify import RegexBuilder

builder = RegexBuilder().start_of_input().exactly(4).digit().end_of_input()
repr(builder)                                # <RegexBuilder '^\d{4}$'>
RegexBuilder().digit() == RegexBuilder().digit()  # True
lookup = {RegexBuilder().digit(): "hit"}          # hashable, dict-key safe
root = RegexBuilder().digit()
branch = root.fork().word()                       # explicit branch off root
```

Closes #136
Closes #137
Closes #139

1 files changed, 50 insertions, 0 deletions

diff --git a/tests/builder/equality.test.py b/tests/builder/equality.test.py
new file mode 100644
index 0000000..717d1ae
--- /dev/null
+++ b/tests/builder/equality.test.py
@@ -0,0 +1,50 @@
+"""Tests for value-based ``__eq__`` and ``__hash__`` on :class:`BuilderCore`."""
+
+from edify import Pattern, RegexBuilder
+
+
+def test_two_builders_with_the_same_chain_are_equal():
+    assert RegexBuilder().digit() == RegexBuilder().digit()
+
+
+def test_two_patterns_with_the_same_chain_are_equal():
+    assert Pattern().digit() == Pattern().digit()
+
+
+def test_builders_with_different_chains_are_not_equal():
+    assert RegexBuilder().digit() != RegexBuilder().word()
+
+
+def test_equal_builders_produce_equal_hashes():
+    assert hash(RegexBuilder().digit()) == hash(RegexBuilder().digit())
+
+
+def test_a_builder_is_usable_as_a_dict_key():
+    lookup = {RegexBuilder().digit(): "digit-key"}
+    assert lookup[RegexBuilder().digit()] == "digit-key"
+
+
+def test_a_builder_can_be_deduplicated_in_a_set():
+    dedup = {RegexBuilder().digit(), RegexBuilder().digit(), RegexBuilder().word()}
+    assert len(dedup) == 2
+
+
+def test_a_pattern_and_a_regex_builder_with_the_same_state_compare_equal():
+    assert Pattern().digit() == RegexBuilder().digit()
+
+
+def test_a_builder_compared_to_a_non_builder_is_not_equal():
+    assert (RegexBuilder().digit() == "foo") is False
+    assert (RegexBuilder().digit() == 42) is False
+
+
+def test_a_builder_compared_to_a_non_builder_returns_not_implemented_from_the_dunder():
+    result = RegexBuilder().digit().__eq__("foo")
+    assert result is NotImplemented
+
+
+def test_hash_of_two_flags_that_differ_are_distinct():
+    a = RegexBuilder().ignore_case().digit()
+    b = RegexBuilder().digit()
+    assert a != b
+    assert hash(a) != hash(b)