feat!: Regex result wrapper on .to_regex(), plus flag kwargs at the terminal (#270)

Reshapes what `.to_regex()` returns and takes, so pattern compilation
becomes a first-class edify value.

## `Regex` result wrapper class
`edify.Regex` is a composition wrapper over `re.Pattern` (which is a
CPython C type and cannot be subclassed). It exposes:
- `.source` — the emitted regex string.
- `.compiled` — the underlying `re.Pattern` for callers that need
identity checks, `isinstance` checks, or direct interop with libraries
typed on `re.Pattern`.
- Eight direct delegates — `.match()`, `.search()`, `.fullmatch()`,
`.findall()`, `.finditer()`, `.sub()`, `.subn()`, `.split()` — mirroring
the `re.Pattern` query surface.
- Value equality on `(source, flags)` plus `__hash__`.

## `to_regex()` now returns `Regex`
The terminal returns `edify.Regex` instead of raw `re.Pattern`. The
existing eight query methods continue to work via delegation. `.match()`
/ `.search()` / … on builders (via `MatcherMixin`) still work because
they go through the same delegation.

**Breaking:** `isinstance(x, re.Pattern)` and `re.Pattern`-typed
annotations on values returned from `.to_regex()` break. Callers that
need the raw pattern read `.compiled`.

## Flag kwargs on `.to_regex(...)`
`.to_regex()` now accepts six keyword-only flag arguments —
`ascii_only`, `debug`, `ignore_case`, `multiline`, `dotall`, `verbose` —
that OR-merge into the flag snapshot the builder already carries.
Passing `ignore_case=True` at the terminal is equivalent to calling
`.ignore_case()` mid-chain. Kwargs never turn a chain-set flag off; the
terminal is the natural home for pattern-global settings.

## Example
```python
from edify import RegexBuilder

email = RegexBuilder().start_of_input().one_or_more().letter().end_of_input()
regex = email.to_regex(ignore_case=True)

regex.source                  # ^[a-zA-Z]+$
regex.match("Hello")          # <re.Match ...>
regex.compiled                # <re.Pattern object; flags=re.IGNORECASE>
```

Closes #133
Closes #134
Closes #128

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