fix: invalid inline-span attribute block stays literal

[dereuromark]

What

A bracketed run directly abutting an attribute block always materialized a <span> and then silently dropped any block that was not valid attribute syntax. The payload vanished:

input	before	after
[x]{???}	<span>x</span>	[x]{???}
[*x*]{???}	<span><strong>x</strong></span>	[<strong>x</strong>]{???}
[x]{.a}{???}	<span class="a">x</span>	<span class="a">x</span>{???}

Fix

A new isValidAttrPayload() gates both span formation and applyConsecutiveAttributes(): a {...} block forms or extends attributes only when it yields an attribute, or is empty/whitespace/comment-only. A block carrying unrecognized content is not an attribute block, so the brackets and the block render literally (the bracket text is still inline-parsed). The change applies to spans, links, and code spans through the shared helper.

Empty and whitespace-only blocks ([x]{}, [x]{ }) still form a bare <span>, kept so DefaultAttributesExtension can target it. Note djot keeps {=...=} highlight, so [x]{=y=} now correctly renders [x]<mark>y</mark> (the highlight was previously swallowed as a span attribute).

Backport of markup-carve/carve-php#43, adapted to djot (which retains {=...=} highlight and the empty-block span).

Known limitation

An exotic [x]{.a}{???}{.b} leaves the trailing {.b} to attach to the now-literal {???} via djot's standalone-attribute feature (text{.b} -> <span class="b">text</span>), yielding <span class="a">x</span><span class="b">{???}</span> rather than fully-literal {???}{.b}. Suppressing that would mean conditionally disabling a separate, legitimate djot feature; out of scope for this fix.

Tests

Added cases for the invalid-block-literal behavior and the preserved empty-block span. Full suite and official corpus green.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.89%. Comparing base (32c0d50) to head (e3a8cbe).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Parser/InlineParser.php	93.75%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #237      +/-   ##
============================================
- Coverage     91.89%   91.89%   -0.01%     
- Complexity     3523     3527       +4     
============================================
  Files           106      106              
  Lines          9958     9970      +12     
============================================
+ Hits           9151     9162      +11     
- Misses          807      808       +1     

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