Match single quotes in one O(n) pass instead of an O(n^2) scan

[dereuromark]

What

InlineParser::buildSingleQuoteMatchCache() decides which single quotes are curly opener/closer pairs and which are apostrophes. It collected potential openers and closers into two lists, then matched each closer against the opener list with a nested scan - O(closers x openers). On quote-heavy text that is quadratic and noticeably slow.

This folds classification and matching into a single forward pass using an opener stack:

if ($prevIsSpace && !$nextIsSpace) {
    $openerStack[] = $i;                      // potential opener
} elseif (!$prevIsSpace && $nextIsSpaceOrPunct && $openerStack) {
    $matched[array_pop($openerStack)] = $i;   // pair with innermost opener
}

The stack top is always the largest-index still-open opener, so popping it reproduces the former "nearest preceding unmatched opener" pairing exactly - now in O(n).

Behavior

Unchanged. The smart-quote semantics, including the official conformance corpus (tests/official/smart.test), render identically; only the cost changes. A new SingleQuoteMatchingTest locks the pairing for matched, lone-opener, nested, and apostrophe-then-pair cases.

This is a perf-only, behavior-preserving change. It deliberately does NOT adopt carve-php's flanking-rule rewrite (which drops opener/closer matching entirely); that diverges from djot.js semantics and would break the conformance corpus. Only the O(n) stack-merge idea is carried over.

Also removes the unused findMatchingSingleQuoteCloser() method, a leftover of the old approach.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.37%. Comparing base (5cf4c4d) to head (695195f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #249      +/-   ##
============================================
+ Coverage     92.10%   92.37%   +0.27%     
+ Complexity     3596     3576      -20     
============================================
  Files           107      107              
  Lines         10167    10129      -38     
============================================
- Hits           9364     9357       -7     
+ Misses          803      772      -31     

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