Summary

Closes #147.

• Adds two mutually-recursive ADTs: MdInline (6 constructors: MdText, MdCode, MdEmph, MdStrong, MdLink, MdImage) and MdBlock (8 constructors: MdParagraph, MdHeading, MdCodeBlock, MdBlockQuote, MdList, MdThematicBreak, MdTable, MdDocument)
• Adds 5 pure functions as WASM host imports: md_parse, md_render, md_has_heading, md_has_code_block, md_extract_code_blocks
• Hand-written Python Markdown parser (~650 lines) with no external dependency — supports ATX headings, fenced code blocks, block quotes, lists, tables, thematic breaks, emphasis, strong, code spans, links, images
• Establishes the host-side binding pattern for pure functions: WASM import interface is the portability contract; any host runtime (Python, JS, Rust) provides matching implementations for the same .wasm binary
• Reorganizes spec §9.3 (Built-in ADTs) to graduate UrlParts and Future<T> from inline definitions
• Fixes _is_void_expr bug (IfExpr attribute names)
• Bumps to v0.0.84

New files: vera/markdown.py (parser/renderer), vera/wasm/markdown.py (WASM marshalling), examples/markdown.vera, tests/conformance/ch09_markdown.vera, tests/test_markdown.py

Test plan

• 2175 tests pass (6 skipped), including ~30 new parser/renderer unit tests, 6 type-checker tests, 8 codegen integration tests
• 52 conformance programs pass (new: ch09_markdown.vera)
• 23 examples pass (new: markdown.vera)
• mypy clean
• All validation scripts clean (spec, README, SKILL block checks)
• Pre-commit hooks pass

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