talents/speaker_attribution: schema-constrain + drop wrapper tolerance
Apply founder decision #3 from the 2026-04-19 audit by schema-constraining speaker_attribution output to a top-level array, loading that schema from prompt frontmatter, and removing post_process tolerance for legacy wrapper shapes; this keeps the migration a clean break per CLAUDE.md §8, adds focused schema and post-hook coverage, and mirrors the new schema field in the stats API baseline.