feat: regex introspection API — explain, verbose export, ASCII + SVG visualize + annotated errors (#272)

Ships the regex introspection API on the `Regex` wrapper — three new
methods that turn a compiled pattern into something a human can read,
plus a full retrofit of every error message to a shape that shows the
caller *where* the failure happened and *how* to fix it.

## `Regex.explain()`

Plain-English bullet list of what the pattern accepts, followed by a
small set of concrete accepted strings.

```
- The text must start with either "http" or "https".
- Then the text must have "://".
- Then the text must have one or more letters, digits, or underscores.

Text this pattern accepts:
    http://ab1
    https://b1_c
    http://1_c2d
```

## `Regex.to_verbose_string()`

An `re.VERBOSE`-compatible export where every fragment is annotated
inline. Copy-pasting the output into `re.compile(..., re.VERBOSE)`
compiles to the same pattern.

```
(?P<year>               # begin group named "year"
  \d{4}                 # exactly 4
)                       # end group named "year"
```

## `Regex.visualize(format='ascii')`

ASCII railroad diagram — `START` and `END` boxes flanking the pattern,
arrows between elements, fork/merge junctions for alternation,
dashed-caption blocks for captures and lookarounds.

```
                        +--------+
                   +--->| "cat"  |----+
                   |    +--------+    |
                   |                  |
   +-------+       |    +--------+    |   +-----+
   | START |------>+--->| "dog"  |----+-->| END |
   +-------+       |    +--------+    |   +-----+
                   |                  |
                   |    +--------+    |
                   +--->| "fish" |----+
                        +--------+
```

## `Regex.visualize(format='svg', engine='graphviz')`

SVG rendered by Graphviz. Junction-point fork/merge for alternation,
dashed rounded clusters for captures/lookarounds, folded two-line node
labels for quantifiers (e.g. `digit\n(one or more)`). Requires the
optional `graphviz` extra; the missing-dependency error names the
install command.

## Annotated error messages

Every exception across `edify.errors` — anchors, captures, input,
internal, naming, quantifier, structure — now emits a message that names
the offending call in the caller's source, explains the invariant that
was violated, and prescribes the fix:

```
error: start_of_input has already been added to this pattern

  --> user_code.py:14:26
   |
14 |     pattern = RegexBuilder().start_of_input().digit().start_of_input()
   |                                                       ^^^^^^^^^^^^^^^^ second start_of_input added here
   |
   = note: a pattern can carry at most one start_of_input anchor; the earlier .start_of_input() call already set it.

help: remove the duplicate .start_of_input() call from the chain.
```

Caller source location is captured via `sys._getframe()` walking to the
first non-edify frame and `co_positions()` for precise column spans.

## Other landings

- `Regex` retains the AST elements the builder produced; every
introspection method operates on the AST directly rather than parsing
the emitted regex string.
- Lazy compile cache on `BuilderCore` — a builder's `to_regex()` result
is memoised so `.match()` / `.search()` / `.explain()` share one
compiled pattern.
- `Regex.__getattr__` delegates any attribute not on the wrapper to the
underlying `re.Pattern`, so `.pattern`, `.flags`, `.groups`,
`.groupindex` continue to work.
- `Regex.__eq__` / `Regex.__hash__` raise when called on an unfinished
builder, pointing at both the compare/hash call site and the still-open
frame or dangling quantifier.
- Property test extended: every recursively-built composition of
quantifiers, groups, captures, named captures, and subexpressions emits
its expected regex exactly, no fragment dropped or duplicated.

1 files changed, 18 insertions, 0 deletions

diff --git a/uv.lock b/uv.lock
index 9e67009..ccd1eb1 100644
--- a/uv.lock
+++ b/uv.lock
@@ -233,8 +233,14 @@ wheels = [
 name = "edify"
 source = { editable = "." }
 
+[package.optional-dependencies]
+graphviz = [
+    { name = "graphviz" },
+]
+
 [package.dev-dependencies]
 dev = [
+    { name = "graphviz" },
     { name = "hypothesis" },
     { name = "pytest" },
     { name = "pytest-cov" },
@@ -247,9 +253,12 @@ docs = [
 ]
 
 [package.metadata]
+requires-dist = [{ name = "graphviz", marker = "extra == 'graphviz'", specifier = ">=0.20" }]
+provides-extras = ["graphviz"]
 
 [package.metadata.requires-dev]
 dev = [
+    { name = "graphviz", specifier = ">=0.20" },
     { name = "hypothesis", specifier = ">=6.100" },
     { name = "pytest", specifier = ">=8.0" },
     { name = "pytest-cov", specifier = ">=5.0" },
@@ -261,6 +270,15 @@ docs = [
 ]
 
 [[package]]
+name = "graphviz"
+version = "0.21"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/f8/b3/3ac91e9be6b761a4b30d66ff165e54439dcd48b83f4e20d644867215f6ca/graphviz-0.21.tar.gz", hash = "sha256:20743e7183be82aaaa8ad6c93f8893c923bd6658a04c32ee115edb3c8a835f78", size = 200434, upload-time = "2025-06-15T09:35:05.824Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/91/4c/e0ce1ef95d4000ebc1c11801f9b944fa5910ecc15b5e351865763d8657f8/graphviz-0.21-py3-none-any.whl", hash = "sha256:54f33de9f4f911d7e84e4191749cac8cc5653f815b06738c54db9a15ab8b1e42", size = 47300, upload-time = "2025-06-15T09:35:04.433Z" },
+]
+
+[[package]]
 name = "hypothesis"
 version = "6.155.7"
 source = { registry = "https://pypi.org/simple" }