Fix: keep trailing content out of an unmatched emphasis closer (#743) (#950)

* fix: keep trailing content out of unmatched emphasis closer (#743)

When EnableTrackTrivia() is enabled, an emphasis closer whose delimiter
run is only partially consumed (its leftover dropping below the minimum
count) kept the inlines that follow it nested inside the emphasis. The
roundtrip renderer then emitted the closing delimiters after that
trailing content, corrupting the output, for example a grid table
separator row using the "marked" (==) delimiter from EmphasisExtras.

Move the trailing content out to the outermost enclosing emphasis when
the closer can no longer be matched, mirroring the existing handling for
a fully consumed opener. The leftover closer keeps its position, so
cases without trailing content are unaffected. Adds a regression test.

* Fix unmatched emphasis closer cleanup order (#743)

---------

Co-authored-by: Alexandre Mutel <alexandre_mutel@live.com>

site/docs/extensions/emphasis-extras.md

@@ -60,6 +60,11 @@ This is ==highlighted== text.
 
 This is ==highlighted== text.
 
+An unmatched remainder of a closing delimiter run must not pull subsequent text
+into the emphasis. For example, `===a===== tail` renders as
+`=<mark>a===</mark> tail`. This example also round-trips unchanged with
+`EnableTrackTrivia()` and `RoundtripRenderer`.
+
 ## HTML output
 
 {.table}