chore!: package reorg + uv/ruff/pytest/hatchling toolchain + Python 3.11 floor (#255)

## Summary

Closes #68 — package reorganization to a single-purpose, dir-per-concern
layout (no `src/`).
Closes #69 — toolchain migration to `uv` + `ruff` + `pytest` +
`hatchling` + `hatch-vcs`, driven entirely from `pyproject.toml`.
Closes #253.
Closes #259 — drop `.editorconfig`; LF / trim / final-newline / Python
indent now enforced by `mixed-line-ending` (added in fixup) + existing
pre-commit hooks + `ruff format`.— Python floor bump to 3.11; drops
`py39` / `py310` / `pypy310` from the CI matrix.

The three issues are paired by construction (dropping `src/` requires
the new build backend's package discovery, and the 3.11 floor lets the
new layout use `typing.Self`, frozen dataclasses, `match`/`case`, and
Python-3.11 idioms throughout).

## Highlights

**Layout** — `edify/builder/` (RegexBuilder + 11 mixin files + `types/`
for state/flags/frame/protocol), `edify/elements/types/` (sealed
dataclass union: 16 leaves, 7 char-shaped, 4 captures, 7 groups, 9
quantifiers, RootElement), `edify/compile/` (per-family render functions
+ dispatch, plus `quantifier/` subpackage for suffix + apply),
`edify/validate/`, `edify/errors/` (typed exception hierarchy),
`edify/library/` (per-validator files in family folders: `email/`,
`ip/`, `date/`, plus flat single-validator files), `pattern/`,
`serialize/`, `result/` skeletons.

**Toolchain** — replaces `tox` / `virtualenv` / `flake8` / `isort` /
`black` / `setup.py` / `setup.cfg` / `pytest.ini` / `.coveragerc` /
`.bumpversion.cfg` / `MANIFEST.in` / `ci/` / `tests.local.sh` with
`pyproject.toml`-driven `uv` + `ruff` + `pytest` + `hatchling` +
`hatch-vcs`. CI workflow rewritten to `uv sync --frozen` + `uv run
pytest`; required-status-check names preserved (`check`, `docs`, `py311
(ubuntu)`, `py312 (ubuntu)`, `py313 (ubuntu)`, `py314 (ubuntu)`,
`pypy311 (ubuntu)`). Pre-commit replaced with `ruff` hooks, all hook
repos pinned by commit SHA.

**Tests** — reorganised under `tests/builder/` and
`tests/library/<family>/` using the `*.test.py` naming shape. 173 tests,
100% line coverage, ruff lint + format clean, `uv build` produces sdist
+ wheel cleanly.

## Behaviour changes versus 0.3

These will get their own upgrade-guide entries when the migration
tooling lands (#80):

- `to_regex_string()` now returns the bare pattern (no `/.../` wrap).
- `to_regex()` on a named back-reference compiles successfully via
`(?P=name)` instead of raising.
- Builder errors raise typed `EdifyError` subclasses
(`MustBeAStringError`, `StartInputAlreadyDefinedError`, etc.) instead of
bare `Exception` — catch via `except EdifyError`.
- Internal helpers (`apply_quantifier`, `frame_creating_element`,
`evaluate`, `state`, etc.) removed outright per the 0.3 → 1.0 stub-free
policy.

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