Implement Integrity Gate + Failure Lifecycle Enforcement

[BLMgithub]

Scope

• Validate semantic directory exists
• Validate expected semantic file set exactly matches required set
• Validate required parquet files exist
• Validate parquet loads successfully
• Validate dataframe not empty
• Validate required schema columns present
• Fail fast on any violation

Failure behavior

• metadata.json → status = FAILED
• completed_at set
• published = false
• terminate execution
• no version directory creation
• no pointer update

Acceptance Criteria

• No incomplete or structurally invalid semantic output can be published
• Gate failure leaves publish layer untouched
• metadata.json reflects FAILED correctly