…neGroup.Lines (#931)

* Fix to calculate LinkReferenceDefinition span positions from StringLineGroup.Lines

* Add test case in TestMultilineInBlockquote

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Change from startPosition + span.End - span.Start to GetAbsolutePosition(lines, span.End)

* Move ConvertToAbsoluteSpan and GetAbsolutePosition to StringLineGroup

* Remove '\r' check in Write(ReadOnlySpan<char>)

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…935) (#936)

* test: add failing regression tests for issue #935 (abbreviation + emphasis)

* fix: AbbreviationParser now substitutes post-inline-parse, fixes emphasis corruption (#935)

The PostMatch hook approach (running mid-parse) produced a ContainerInline
with IsClosed=false that disrupted FindLastContainer(), causing emphasis
delimiters to be appended as children of the abbreviation container instead
of as root siblings. It also had an off-by-one (i != 0 vs i != content.Start)
that caused abbreviations to be missed when the literal started inside emphasis.

Replace with a document.ProcessInlinesEnd handler that walks the fully resolved
inline tree (after EmphasisInlineParser and LinkInlineParser have run), inserting
AbbreviationInline/LiteralInline siblings directly into parent containers without
any wrapper ContainerInline.

Also update TestSourcePosition.TestAbbreviations to reflect the new tree shape
(no container wrapper; empty leading literals at word boundaries are pruned).

Fixes #935

Introduces AllowNestedAlerts on AlertExtension and AlertInlineParser
(default: false) to allow alert blocks inside blockquotes or list items.

The UseAlertBlocks() pipeline extension method exposes the new option
via an allowNestedAlerts parameter.

Resolves #853

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>