[wxs-lang]

The existing suite (tests/unit_test.py, fixture files and doctests) covers loading the bundled examples and load → dump → load round trips. This PR adds pytest coverage for the parts of the public API that had none, with all assertions taken from the library's current behaviour:

• frontmatter/util.py (previously untested) — u() normalises bytes/str to unicode and CRLF → LF, and stringifies non-string input; is_readable / is_writable / can_open classification of file-like objects, paths and other values
• parse() semantics — the (metadata, content) tuple, keyword defaults being used and overridden by front matter, non-dict YAML front matter (lists/scalars) being silently ignored, unclosed or non-leading front matter treated as plain text, and a handler whose split raises ValueError bailing out to defaults + untouched text
• check / checks / detect_format — string inputs, file-like objects, detection order (YAML → JSON → TOML), and unopenable argument types
• Error paths — load/dump with an unopenable argument raise ValueError; invalid YAML/JSON/TOML front matter propagates the underlying parser errors instead of being swallowed
• YAML specifics — scalar type coercion (int/float/bool/null/list/nested map), unquoted dates becoming datetime.date, quoted numbers staying strings, anchors/aliases, delimiter variants (----, trailing spaces), later --- lines staying in the content, export key sorting, and the sort_keys=False passthrough in dumps
• JSON & TOML handlers — detection and round trips with nested structures, TOML datetimes and tables, JSON export indentation/key order, and handler override in dumps changing the output format
• Post — the full metadata mapping protocol ([], in, set/delete, get, keys, values), to_dict() returning a copy with content added, str/bytes, and the handler fallback to YAML when post.handler is None
• Custom handlers — a real BaseHandler subclass (a ;;;-delimited key = value format) driven end to end, registration in frontmatter.handlers, detection precedence, and BaseHandler raising NotImplementedError without a boundary

Current-behaviour quirks pinned by tests (not changed):

• A one-line JSON document ({"a": 1}) is not detected as front matter, because the JSON handler's boundary only matches a line that is exactly { or }
• frontmatter.check() on a missing file raises FileNotFoundError instead of returning False
• Dumping an empty-metadata post produces ---\n{}\n--- (YAML), {} plus an extra blank line from the shared template (JSON), and +++\n\n+++ (TOML)
• Post(None).content is the string "None" (content is coerced with str())

All tests live in new files under the existing tests/ directory; no production code is changed. The TOML tests skip cleanly if the optional toml package is not installed. No new conftest.py is needed: the package is installed by the project's test setup (uv sync / the test extra, which also provides toml).

Verification (Python 3.12, PyYAML + toml installed):

$ python -m pytest tests/test_util.py tests/test_parse_api.py tests/test_yaml_format.py tests/test_json_toml_formats.py tests/test_post_behavior.py tests/test_custom_handlers.py -q
79 passed
$ python -m pytest . --doctest-modules --doctest-glob "README.md" -q   # full suite as CI runs it
146 passed