docs(spec): place CompressionEvaluation in sourceos-spec contract lane

[mdheller]

Summary

This PR adds a docs-only placement decision for Compression Commons in sourceos-spec.

It does not add schemas yet. Instead, it records the contract-lane decision and the exact first schema/API landing plan so follow-on work can happen in the right repo and in the right order.

Added

• docs/adr/0010-compression-evaluation-placement.md
• docs/contract-additions/compression-evaluation-v0-placement.md

Decision captured here

• sourceos-workspace remains planning/freeze only
• sourceos-spec is the machine-readable contract canon
• Compression Commons should enter as one new top-level metadata-plane schema: CompressionEvaluation
• Reuse existing PolicyDecision, RunRecord, ProvenanceRecord, ContentRef, and DataRef
• First API addition should be POST /v2/compression-evaluations/evaluate
• No AsyncAPI or agent-plane additions in v0

Why docs-first

There is active upstream drift around contract ownership across sourceos-workspace, source-os, and socioprophet-agent-standards. This PR resolves placement before we write schema code.