feat(markdown): add text transform helper

[cixzhang]

Builder impact

Markdown builders can replace matched prose with typed Markdown nodes without preprocessing source or entering protected content such as code, links, images, citations, math, or prior extension syntax.

Depends on #6338, #6339, and #6340.

What changed

• adds createMarkdownTextTransform() as a helper that compiles to the core immutable transform protocol;
• preserves structural sharing when no eligible text changes;
• supports ordered replacement, removal, bounded claim extension/decline, and parent-kind context;
• adds optional exact requiredSubstrings prefilters so hinted no-claim helpers skip AST traversal;
• adds a TODO demo plugin to the shared Markdown composition story.

AST-036 scope

This PR implements the text-helper portions of FR1, FR6, FR7, FR10–FR13, and FR21. It adds the hinted no-claim fast-path evidence needed by FR23; the complete representative and hintless helper budget remains for the combined helper stack after semantic-fence and source-decoration support land.

Validation

• Core TypeScript check
• changed-file ESLint and Prettier
• 372 non-performance Markdown/Outline tests
• 18 parser/helper performance tests
• exact 200/500-section alternating paired benchmark: stable and recreated lists of five hinted no-claim helpers stayed within the 1.15 ratio
• production Storybook build

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Project	Deployment	Actions	Updated
astryx	Ready	Preview	Sep 20, 2026 8:39am UTC

[astracat-bot]

AI review status for this pull request.

Review status	Updated
🟢 Reviewed (for maintainers only)	Sep 16, 2026, 8:12 AM UTC