fix: keep comment anchors within Markdown block content

[petergaultney]

🍋:

Comment anchors can still include Markdown structure when a selection crosses into another block or when selected text changes while the draft is open. This extends the existing prefix narrowing: comments keep closing heading hashes and selected line breaks outside their markup, and saving validates the current mapped range again. Prefixes remain selectable.

• Refuse selections that cross into a later prefixed line, including selections starting in ordinary prose or ending within a later prefix.
• Refuse prefixed selections spanning multiple source lines, including continuation lines within a list item. The README documents that limitation.
• Support nested lists with tabs or spaces using the shared structuralPrefixLength parser, including combined quote/list containers. Keep indentation outside the mark without trying to infer whether a line is code or list content. Use a clearer notice when a pending draft's text becomes unsafe to wrap.
• Preserve the existing native-highlight and overlap checks. The source-based helper adds no parser dependency and does not replace the existing selection behavior.

Validation: 140 focused tests across five suites, TypeScript check, lint, no-node check, production build, and whitespace checks passed on this upstream-based branch. Coverage includes ordered/unordered lists, tasks, headings, blockquotes, LF/CRLF, selection direction, native highlights, mapped edits, cross-block ranges, and nested list/quote and code-text contexts.

Diff breakdown

Category	Files	+Lines	-Lines	Net	Total	% of diff
code	5	+269	-17	+252	286	97.9%
docs	1	+6	-0	+6	6	2.1%