✨ Add an accordion component

[chrisjsewell]

Adds an accordion directive: a group of dropdowns where opening one closes the others — with zero JavaScript. The directive stamps a shared, deterministic name attribute onto its direct child dropdowns' <details> elements, and the browser's native exclusive-open behaviour does the rest.

.. accordion::

   .. dropdown:: First item

      Content

   .. dropdown:: Second item
      :open:

      Content

Behaviour

• Only direct child dropdowns join the group — a dropdown nested inside an item's content stays independent (tested).
• At most one :open: item: a second is stripped from the output with a suppressible design.accordion warning.
• Non-dropdown children warn (design.accordion) and are dropped, except inert hyperlink targets, which are kept so :ref: anchors keep working.
• :flush: renders the group edge-to-edge; custom directives can inherit accordion via sd_custom_directives (tested).
• Group names are sd-accordion-<slug>-<digest>-<serial> — the digest of the raw docname guards against slug collisions (a/b vs a-b) coupling unrelated groups in singlehtml/concatenated builds, which review caught with a browser-verified reproduction and is now locked in by a dedicated singlehtml test.

Browser support

<details name> is Baseline Newly Available (Chrome/Edge 120+, Firefox 130+, Safari 17.2+) — a documented per-feature exception to the support policy: older browsers simply ignore the attribute and render ordinary independently-collapsible dropdowns. Nothing breaks, no JS fallback involved.

Verification

• Native exclusivity verified in real Chromium (Playwright): click and keyboard (Enter/Space) toggling closes siblings; nested dropdowns unaffected
• Plain dropdowns are byte-identical to main (the dropdown.py visitor change is a strict no-op without the attribute)
• Full test/lint/type matrix green; docs -nW; the CSS artifact regenerated through the generator pipeline (style/accordion.css registered in ASSEMBLY after dropdown.css)

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.63%. Comparing base (4f66f32) to head (b98b166).

Files with missing lines	Patch %	Lines
sphinx_design/accordion.py	91.48%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #291      +/-   ##
==========================================
- Coverage   92.63%   92.63%   -0.01%     
==========================================
  Files          13       14       +1     
  Lines        1371     1425      +54     
==========================================
+ Hits         1270     1320      +50     
- Misses        101      105       +4     

Flag	Coverage Δ
pytests	92.63% <92.85%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.