♻️ Serve static assets via html_static_path

[chrisjsewell]

This replaces the hand-rolled static-asset handling (update_css_js + update_css_links) with Sphinx's standard html_static_path mechanism, registered once on builder-inited and gated to HTML-format builders.

Changes

• CSS/JS moved (git mv, contents unchanged) from sphinx_design/compiled/ to a dedicated sphinx_design/static/ dir, served under the same output names (sphinx-design.min.css, design-tabs.js) — HTML output is byte-identical to main
• Non-HTML builders no longer gain a spurious _sphinx_design_static directory in the output, and nothing is written into outdir outside Sphinx's own copying, which also removes the mkdir crash on Sphinx ≥8.1/Windows
• Sphinx floor raised to >=7.2: native ?v= cache-busting replaces the old md5-in-filename scheme, and the test-suite sphinx_path compat branch is dropped
• The private sphinx_design._compat.findall shim is removed; all call sites now use node.findall(...) directly
• epub behaviour is unchanged (Epub3Builder.format == "html", so it keeps receiving the assets, as before)

Verification

• New tests/test_assets.py: the latex test was confirmed failing against the previous implementation (reproducing the spurious static dir); the html test asserts _static assets exist and are linked with ?v= checksums; plus an epub smoke test
• Full suite passes locally on Sphinx 7.2.6 / 7.4.7 / 8.2.3 / 9.0.4 / 9.1.0 (docutils 0.20–0.22); builder-by-builder output audit (html/dirhtml/singlehtml/epub/json/latex/text/man/texinfo/gettext) matches main for HTML-family builders and is clean for the rest
• Wheel/sdist inspected: the only difference from main is the two renamed asset paths; icon data stays in sphinx_design/compiled/

Closes #200
Closes #235
Supersedes #241

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.33%. Comparing base (9f62c7d) to head (7a3f261).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #276      +/-   ##
==========================================
+ Coverage   90.08%   90.33%   +0.25%     
==========================================
  Files          12       12              
  Lines        1119     1086      -33     
==========================================
- Hits         1008      981      -27     
+ Misses        111      105       -6     

Flag	Coverage Δ
pytests	90.33% <100.00%> (+0.25%)	⬆️

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