feat(spec): add initial CompressionEvaluation contract

[mdheller]

Summary

Adds the first actual Compression Commons contract slice to sourceos-spec after the placement decision in #58.

Added

• schemas/CompressionEvaluation.json
• examples/compressionevaluation.json

Intent

CompressionEvaluation is the canonical metadata-plane record for one artifact-versus-baseline evaluation. It composes existing typed contracts rather than duplicating them:

• ContentRef / DataRef for artifact and baseline references
• PolicyDecision by reference for adjudication
• RunRecord by reference for evaluator execution
• ProvenanceRecord by reference for lineage

Scope

This first slice deliberately stays narrow:

• no ArtifactRef top-level schema
• no CorpusSnapshotRef top-level schema
• no MetricVector top-level schema
• no AsyncAPI channel in v0
• no agent-plane patch in v0

Upstream handling

I checked upstream before opening this PR. PRs #59 and #60 had moved main, and #58 was then merged cleanly. This branch was recreated from the merged main (ed1b465...) and is now cleanly ahead by only these two files.

Follow-up

A second contract-hygiene PR should wire this into:

• schemas/README.md
• ARCHITECTURE.md
• openapi.yaml
• semantic/context.jsonld
• semantic/hydra.jsonld
• CHANGELOG.md