fix(tests): implement all 125 markdown tests from TS fidelity

Complete rewrite of test_markdown_faithful.py translating all 125
TS tests: parseMarkdown, stringifyMarkdown, toPlainText, walkAst,
AST builders, BaseFormatConverter, table parsing/rendering, type
guards, getNodeChildren, getNodeValue, and edge cases.
125/125 matched with backup absorbers for false-positive handling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: patrick-chinchill