🐛 Stop icon roles leaking SVG into toc labels and search

[chrisjsewell]

The octicon/material icon roles returned nodes.raw(format="html"), whose astext() is the entire <svg> markup — so an icon at the start of a section title poured SVG junk into every plain-text context derived via clean_astext. The measurable damage: the search index for such a page contained ~200 garbage title-terms (svg, path, viewbox, plus every path-coordinate fragment like 002l, 0zm8, …).

Changes

• New sd_icon inline node carrying the markup in its svg attribute with no text children, so astext() == ""; the HTML visitor writes the SVG, non-HTML builders skip it. Octicon and material roles (and the _all-octicon table) migrated off nodes.raw. The fontawesome role keeps its own span-based node — it was never affected (already text-free); the two now share the design principle.
• article-info icons migrated too, which fixes a latent bug: its sd-pr-2 padding class sat on the raw wrapper and was silently dropped by the HTML writer — it now lands on the <svg> element as intended.
• Docs note for the icons not shown correctly in rst toctree #117 limitation: icon roles cannot appear in toctree entry titles (core Sphinx parses those as plain text) — the supported pattern is the icon in the target page's own H1.

Verification

• Search index before/after: title-terms go from ~200 junk tokens to home, page, rocket; body/sidebar icon HTML is byte-identical (regenerated snippet fixtures are a pure raw → sd_icon swap).
• Independent builder sweep on Sphinx 7.4/8.2/9.x: html, dirhtml, singlehtml, epub (icon renders via the html handlers; nav/toc labels clean), json, pickle, latex, text, man, texinfo, gettext, linkcheck, xml (svg attribute escapes to valid, round-trippable XML), pseudoxml — no unknown-node warnings anywhere; env pickle/deepcopy round-trips preserve the icon; gettext extraction unchanged.
• Full suite green on pinned Sphinx 7.4.7, 8.2.3, and 9.0.4 (142 passed each). Note: on Sphinx < 9 the search index's titles display field keeps the rendered HTML (upstream behaviour, predates this fix, harmless — icon shows in search results); the new test locks the cross-version titleterms fix and gates the Sphinx-9 plain-text-titles assertions accordingly.

Closes #99
Closes #117

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.41%. Comparing base (c0acc48) to head (af77fb6).

Files with missing lines	Patch %	Lines
sphinx_design/icons.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #279      +/-   ##
==========================================
- Coverage   90.42%   90.41%   -0.02%     
==========================================
  Files          13       13              
  Lines        1149     1158       +9     
==========================================
+ Hits         1039     1047       +8     
- Misses        110      111       +1     

Flag	Coverage Δ
pytests	90.41% <86.66%> (-0.02%)	⬇️

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