[starter] Add a versioned schema compatibility test for run evidence

[ashishki]

Problem

Runtime Grid emits versioned JSON plus a checksum manifest, but consumers need an explicit compatibility fixture for the machine-readable run-evidence contract.

Bounded scope

Add a checked-in schema or equivalent strict compatibility validator for the current agent-runtime-grid.run-evidence.v1 output.

Acceptance criteria

• A deterministic generated fixture validates against the current contract.
• Negative tests reject missing lifecycle totals, component revision, environment/proof mode, artifact hashes, and terminal-invariant fields.
• Documentation defines additive-compatible changes versus a schema-version bump.
• The existing checksum/content-address verifier remains authoritative for artifact bytes.
• CI uses only local stub data; no generated report with absolute paths is committed.

Non-goals

No dashboard, hosted API, new queue backend, or production-SLO claim.