[v1.0] Add golden cross-language contract fixtures and validators

[codeforester]

Goal

Prevent accidental drift between documented JSON/NDJSON/protocol contracts and real serialized output.

Scope

• Create a versioned golden corpus for success, error, inspection, log, NDJSON, and command-protocol cases.
• Validate fixtures with the published schemas.
• Add a language-neutral validation job and at least one non-Python reference validator.
• Test forward/backward compatibility rules for supported schema versions.

Acceptance criteria

• Golden fixtures cover normal, boundary, redaction, and failure cases.
• CI detects serializer or schema changes that alter the contract unexpectedly.
• Fixtures can be consumed without base-cli internals.
• Compatibility failures produce actionable diffs.

Project fields

• Status: Backlog
• Priority: P1
• Area: CI
• Initiative: v1.0 Readiness
• Size: M