test: add real-world ADR corpus integration tests

Tests parser against ADR files from 8 open-source projects covering
format variations found in the wild: adr-tools (Nygard), MADR 4.0.0,
Decentraland (EIP-style with SPDX), Backstage (Docusaurus), PMD
(Jekyll), log4brains (MADR 2.x), and Buildbarn (freeform H1 sections).

Exercises edge cases: missing title in frontmatter (#186), unknown YAML
fields, YAML comments, Jekyll nav_order metadata, no H1 heading, custom
status values, and title appearing in both frontmatter and body.

Author: joshrotenberg