Formal spec at docs/architecture/line-pool-format.md defining YAML
structure for dialogue and monologue content files. Covers tag enums,
4-layer filtering pipeline, prerequisite-to-KG mapping, ID format,
validation rules, and Rust loader interface.
Fixes monologue schema: adds required constraint on relationship
prerequisite target/state fields.
Ref: D-028, D-032, D-035, D-041
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>