feat: add price change event foundation

[linqiang-max]

Summary

Adds a trusted Price Change Events foundation for tracking verified AI API pricing changes over time.

Changes

• Added a canonical price-change-event schema
• Added snapshot diff generation for:
  • price_update
  • cached_price_added
  • cached_price_removed
• Added stable event_id and dedupe_key
• Added validation for dates, prices, URLs, source snapshot paths, and duplicate events
• Added atomic JSONL writes and idempotent event generation
• Added canonical events for:
  • Mistral mistral-large
  • xAI grok-4.3
• Added unit tests and README documentation

Data integrity

• Does not infer or invent historical prices
• Does not treat verification timestamp changes as price changes
• Preserves unknown provider effective dates as null
• Uses detected_at, verified_at, and date_basis separately
• Repeated generation does not create duplicate events
• Existing manually added metadata is preserved

Validation

• python scripts\validate.py
• python scripts\validate_pricing_v2_preview.py
• python -m unittest tests.test_price_change_events
• git diff --check

All checks passed.

Current coverage

• Verified price change events: 2
• Providers covered: 2
• Public Pricing History page is not included in this PR
• More verified events are still required before publishing a public Pricing History page