♻️ Central declarative SdConfig dataclass

[chrisjsewell]

Foundation for #265 §3.9: all global configuration is now declared once on a SdConfig dataclass (sphinx_design/config.py), modelled on myst-parser's MdParserConfig + dc_validators pattern — per-field type, default, validator, and help text in field metadata, with every value TOML-representable (so config could later be read from a TOML file, or understood by non-Python implementations).

• Flat sd_* confvals remain the public interface, registered by iterating the dataclass fields; modules read via a typed get_sd_config(env) accessor.
• The two existing options (sd_custom_directives, sd_fontawesome_latex) migrate with unchanged names, defaults, and semantics for valid configs; validation warnings unify under design.config (suppressible), running before Sphinx core's confval type check so users get one clear message.
• Invalid sd_custom_directives entries are now warned about and dropped individually (previously some invalid shapes passed silently or half-registered; dropping also removes a latent crash when an invalid entry overrode a built-in directive name). Valid configs are byte-identical in behaviour.
• Docs: "Global options" table auto-generated from the dataclass fields.
• Tests: per-field invalid-type fallback (parametrized over the dataclass fields with a completeness guard), entry pruning, suppress_warnings support, and a TOML round-trip via stdlib tomllib covering every field.
• Drive-by fix: the rst/myst-parametrized tests from 🐛 Ignore comments/targets in component child validation #271 broke the (non-CI) py311-no-myst tox env — the myst variants now carry the same skipif guard as test_snippets.

Upcoming options from the roadmap (tabs storage prefix, FontAwesome config, card legacy-syntax flag) land as fields here.

Validation at head: tox -e py311 (all passing incl. #270's i18n tests), py311-no-myst (fixed), mypy, ruff check/format, docs-alabaster — all green. Reviewed line-by-line (full diff) before merge per the new review gate.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.95%. Comparing base (834eebe) to head (75801ac).

Files with missing lines	Patch %	Lines
sphinx_design/config.py	89.47%	10 Missing ⚠️
sphinx_design/icons.py	66.66%	1 Missing ⚠️
sphinx_design/shared.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
+ Coverage   88.39%   89.95%   +1.55%     
==========================================
  Files          11       12       +1     
  Lines        1043     1115      +72     
==========================================
+ Hits          922     1003      +81     
+ Misses        121      112       -9     

Flag	Coverage Δ
pytests	89.95% <88.88%> (+1.55%)	⬆️

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