The hand-rolled MD parser is replaced with a markdown-it token-stream
walker. AstTable and AstCodeBlock are dropped from the AST — the
substrate doesn't address into table rows or fence content, and
markdown-it's tokenizer already handles "##/- inside fenced code
should not be a heading/item" correctly without first-class AST
modeling.
Grammar opinions move from parser to lint:
- Indented `## foo` (1-3 spaces) is now a heading
- Empty `## ` is a heading with empty slug
- Ordered lists (`1. step`) become items
- Nested sub-bullets become items at flat level
Each was previously a silent parser refusal — now they are recognized
shapes. Lint rules can flag them (`OC_HEADING_INDENTED`,
`OC_HEADING_EMPTY`, etc.) where authoring conventions require the
narrower shape.
Net: parse.ts drops 301 → 207 LoC; tables/code-blocks scenario tests
removed wholesale (-251 LoC of test surface that pinned dead AST
fields).
7b7e65105b