🐛 Fix button-ref target whitespace and nested content

[chrisjsewell]

Two button-ref bugs, plus a parity improvement:

Changes

• :ref: does not resolve autosectionlabel #110 — targets no longer whitespace-stripped: both button directives ran their argument through directives.uri, which deletes all whitespace — correct for button-link URLs, but it turned multi-word ref targets (exactly what autosectionlabel generates, e.g. my article section) into myarticlesection → undefined label. Each directive now normalizes its own argument: button-link keeps directives.uri; button-ref collapses whitespace runs like Sphinx's XRefRole does. Additionally, ref-type: ref targets are lowercased to match the :ref: role (XRefRole(lowercase=True) — std labels are stored lowercased), so the visible Title-Case heading text can be pasted verbatim; doc/myst targets stay case-sensitive.
• Nested parse broken for button-ref but not button-link #228 — rich content preserved through resolution: the std-domain resolvers rebuild explicit-title refs from astext(), flattening nested inline markup (emphasis, icons rendered as escaped text). Fixed with a stash/graft pair of post-transforms using only stable APIs: content is stashed and the pending_xref tagged with a transient marker class before any resolver runs (priority 8 — below myst-parser's resolver at 9 and the built-in at 10), then grafted back onto the resolved reference (priority 11). Buttons whose content itself contains a cross-reference role are deliberately left on the old flatten path — regrafting would re-insert an unresolved pending_xref and crash the writer (and a link inside a button link is invalid HTML anyway).
• Content-less button-refs keep the resolver-generated section title (guarded by refexplicit).
• Unresolved button-refs degrade to a standard undefined label warning with unlinked content — no traceback (locking in the hardening from 🐛 Keep buttons intact under gettext translation #270). Docs updated: the "bold text will be rendered without bold" caveat is gone (it now renders), and targets may contain spaces.

Verification

• New tests/test_buttons.py (11 tests, rst + myst): multi-word target resolution (verified red on the old code: undefined label: 'myarticlesection'), Title-Case + ref-type: ref, nested-content parity with button-link, nested-xref-in-content build safety (verified red — reproduced the writer crash pre-fix), content-less title rendering, unresolved warning path, and marker-leak assertions (HTML + doctree) on every path.
• Independent adversarial review: marker-leak sweep across HTML/latex/text/xml/gettext outputs and pickled doctrees/env (clean, including incremental rebuilds), resolver-path matrix (std ref/doc, any, myst resolving + failing, missing-ref), sd-btn class output byte-identical to main outside the intended fixes, nitpicky-mode parity.
• Full suite green on pinned Sphinx 7.2.6 / 7.4.7 / 8.2.3 (145 passed each, versions verified in-venv) and the standard matrix on 9.0.4. An earlier draft used a private resolver API that doesn't exist before Sphinx 8.2 — the final design deliberately uses only stable public mechanics.

Closes #110
Closes #228

[codecov]

Codecov Report

❌ Patch coverage is 98.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.72%. Comparing base (220407d) to head (926cc8b).

Files with missing lines	Patch %	Lines
sphinx_design/badges_buttons.py	98.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #281      +/-   ##
==========================================
+ Coverage   90.41%   90.72%   +0.30%     
==========================================
  Files          13       13              
  Lines        1158     1207      +49     
==========================================
+ Hits         1047     1095      +48     
- Misses        111      112       +1     

Flag	Coverage Δ
pytests	90.72% <98.00%> (+0.30%)	⬆️

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