The serializer escapes a marker run too short to open a thematic break

[Azganoth]

Summary

Saving escapes a ** written on its own line, where CommonMark needs three or more markers to open a thematic break. The line is literal text as written and as escaped, so the document is unchanged, but the file gains two backslashes the author did not write. corpus/commonmark/blocks.md carries the line under the heading Too few or unsupported markers remain text, which is the property being measured.

Steps to reproduce

1. Open a document whose own line holds **.
2. Save it without editing.
3. Read the file.

Expected behavior

**, unchanged. Two markers cannot open a thematic break, so nothing needs holding literal, as ++ + two lines below it already stays bare.

Actual behavior

Measured against 8bc36754 by driving the editor mount used by the plugin tests, on corpus/commonmark/blocks.md line 18.

Document	Saved
**	\*\*

Related context

• Related issues or pull requests: Preserve the authored form of a Markdown file on save #251 is the parent. The serializer escapes markers where the construct they open cannot form #301 is the closest precedent, relaxing a block marker escaped where the block cannot start; The serializer escapes a run the multiple-of-three rule already leaves literal #372 reaches the same family through the pairing rule rather than through the block-construct check.
• Documentation, decisions, or prior investigation: docs/decisions.md under Preserve the form a file was written in; corpus/commonmark/blocks.md under Too few or unsupported markers remain text.

Done when

• A marker run too short to open a thematic break is written without an escape.
• A run that can open one still keeps what it needs, and ***, ---, and _ _ _ still round-trip as thematic breaks.
• Regression coverage asserts the saved bytes and the document the saved file reopens as, since closure already holds and cannot catch this.

Notes, logs, screenshots

Diagnosis

opensBlockConstruct in src/features/editor/utils/markdownText.ts tests the line's character class without its length:

const thematicBreak = THEMATIC_BREAK_PATTERNS[run.character].test(line);

THEMATIC_BREAK_PATTERNS is { "*": /^[*\t ]*$/u, _: /^[_\t ]*$/u }, which ** satisfies, so a run too short to open the construct is treated as one that could and keeps its escape. The tilde branch a few lines above already makes the length check this pattern omits, returning run.end - run.start >= 3.

An underscore run reaches the same check through the _ pattern and behaves identically, measured against 8bc36754: _ is written \_ and __ is written \_\_. The length test that relaxes ** relaxes both, so this issue covers the underscore as well as the asterisk.

A single * is not affected, because nothing escapes it in the first place and these passes only remove escapes.

Out of scope

• The -- two lines below in the same file, which is also escaped on save. A hyphen is not in ATTENTION_CHARACTERS, so it never reaches this check and is held by a different path whose cause is not established.
• The escaped backticks under Block structure takes precedence over inline parsing in the same file, which are a code span crossing a list-item boundary rather than a block marker.