🐛 Fix card 🔗 whitespace stripping for ref targets

[chrisjsewell]

Follow-up to #281: issue #110's literal reproduction used grid-item-card :link: with link-type: ref, and that path was still broken — both card and grid-item-card ran the link option through directives.uri, which deletes all whitespace (my section name → mysectionname → undefined label). Notably grid-item-card has its own option spec, verified to need its own fix.

Changes

• link option becomes unchanged_required; normalization happens in create_card where link-type is known: url → directives.uri (byte-identical to before — verified across spaces/%20/escapes/tabs), ref → whitespace-collapse + lowercase (parity with the :ref: role and 🐛 Fix button-ref target whitespace and nested content #281's button-ref), doc/any → whitespace-collapse only (case-sensitive targets).
• The visually-hidden fallback link text now shows the raw value rather than the whitespace-stripped one (cosmetic improvement; nothing asserted the old form).

Behaviour note: a target that only resolved because stripping accidentally matched a spaceless label (e.g. label mysectionname referenced as my section name) no longer resolves — that relied on the bug.

Verification

• New tests/test_card_link.py (8 tests, rst + myst): multi-word ref targets for both directives (verified red on main with the exact undefined label: 'mysectionname' warnings), Title-Case lowercase parity, URL byte-parity, unresolved-ref degradation.
• Independent review: URL path byte-identical to main for every URI shape probed; empty-value error behaviour unchanged; no other directives.uri link sites remain; existing card/grid fixtures untouched.
• Full suite green on pinned Sphinx 7.2.6 / 7.4.7 / 8.2.3 (176 passed each, versions verified in-venv).

Completes the fix for #110

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.81%. Comparing base (abbf459) to head (ba3f244).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
+ Coverage   90.72%   90.81%   +0.09%     
==========================================
  Files          13       13              
  Lines        1207     1219      +12     
==========================================
+ Hits         1095     1107      +12     
  Misses        112      112              

Flag	Coverage Δ
pytests	90.81% <100.00%> (+0.09%)	⬆️

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