[IMPL-002] Implement data package (PJM loader, OpenMeteo client, feature engineering)

[h0ffmann]

Type

implementation | Priority: p0

Depends on

• #IFACE-004
• #IFACE-009
• #IFACE-010
• #IMPL-001
• #DATA-001

Scope

• Touch: packages/data/src/data/ (all files)
• Do not touch: any other package

Description

Implement load_pjm(), fetch_weather(), and build_features(). Also implement scripts/download_pjm.py CLI (currently missing from repo). Resolve the 2022–2024 data source issue from DATA-001.

Acceptance Criteria

• load_pjm() handles DST deduplication as decided in DATA-001
• fetch_weather() returns all OPENMETEO_HOURLY_VARS for each region
• build_features() produces all tier-1, tier-2, tier-3 columns per catalog
• validate_dataset() passes on the joined load + weather DataFrame
• openmeteo-requests SDK used (FlatBuffers binary encoding, faster than JSON)