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, 47 insertions, 0 deletions

diff --git a/tests/builder/fork.test.py b/tests/builder/fork.test.py
new file mode 100644
index 0000000..bcb8d1b
--- /dev/null
+++ b/tests/builder/fork.test.py
@@ -0,0 +1,47 @@
+"""Tests for the :meth:`BuilderCore.fork` and :meth:`BuilderCore.copy` aliases."""
+
+from edify import Pattern, RegexBuilder
+
+
+def test_fork_returns_a_regex_builder_with_the_same_state():
+    original = RegexBuilder().digit()
+    forked = original.fork()
+    assert forked == original
+    assert forked is not original
+
+
+def test_fork_returns_a_pattern_with_the_same_state():
+    original = Pattern().word()
+    forked = original.fork()
+    assert forked == original
+    assert forked is not original
+
+
+def test_copy_returns_a_regex_builder_with_the_same_state():
+    original = RegexBuilder().digit()
+    copied = original.copy()
+    assert copied == original
+    assert copied is not original
+
+
+def test_copy_returns_a_pattern_with_the_same_state():
+    original = Pattern().word()
+    copied = original.copy()
+    assert copied == original
+    assert copied is not original
+
+
+def test_fork_and_copy_produce_independent_branches():
+    root = RegexBuilder().digit()
+    branch_a = root.fork().word()
+    branch_b = root.copy().whitespace_char()
+    assert root.to_regex_string() == "\\d"
+    assert branch_a.to_regex_string() == "\\d\\w"
+    assert branch_b.to_regex_string() == "\\d\\s"
+
+
+def test_fork_and_copy_return_the_concrete_class_type():
+    assert isinstance(RegexBuilder().fork(), RegexBuilder)
+    assert isinstance(RegexBuilder().copy(), RegexBuilder)
+    assert isinstance(Pattern().fork(), Pattern)
+    assert isinstance(Pattern().copy(), Pattern)