Add player_bio_download notebook for biographical enrichment

[colettace]

Motivation

• Provide a standalone, re-runnable raw-enrichment workflow that extracts player biographical attributes separate from ranking logic.
• Create a stable, versionable player-bio layer to power downstream joins and lookups.
• Ensure a consistent schema and basic data quality checks so outputs are reliable for downstream pipelines.

Description

• Add notebooks/player_bio_download.ipynb, a notebook that loads a player universe from pybaseball leaderboards and optionally the latest status roster snapshot.
• Map Fangraphs ↔ MLBAM IDs using chadwick_register, then batch-fetch player bio fields from the MLB Stats API /people endpoint with configurable PEOPLE_BATCH_SIZE and basic HTTP/URL error handling.
• Normalize to a stable schema with columns mlbam_id, fangraphs_id, full_name, birth_date, computed age, bats, throws, birth_country, birth_city, and debut_date, deduped by mlbam_id.
• Add basic quality checks for per-column missing rates and handedness domain validation (bats ∈ {L,R,S}, throws ∈ {L,R}), and persist output to data/player_bio_<DATE_TAG>.parquet.

Testing

• Validated notebook JSON syntax with python3 -m json.tool notebooks/player_bio_download.ipynb, which completed successfully.

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Codex Task