F2: Add negative-result archive validator

[cschanhniem]

Bottleneck

Negative results can be submitted without structure, making them unhelpful for learning.

Task

Add validator script that checks negative-result entries against the schema in NEGATIVE_RESULT_ARCHIVE.md. Required fields: candidate_id, sequence, failure_type, evidence_level, timestamp.

Acceptance criteria

• Validates required fields
• Rejects entries with missing mandatory fields
• CLI: openamp-foundry validate-negative-archive <path>
• 4+ tests

Source

NEXT_100_PR_MAP, F2

[cschanhniem]

Negative-result schema exists at schemas/negative_result_entry.schema.json with tests.