x07-data-interop-lab

Reference implementation for Prompt 3 in docs/agent/readiness-checks.md.

• Prompt: PROMPT.md
• Run: x07 run --stdin --profile sandbox
• Tests: x07 test --manifest tests/tests.json
• Stream reader smoke (sandbox): x07 test --manifest tests/tests.json

This implementation focuses on deterministic codec interoperability checks in solve-pure and adds one sandboxed ext.streams file-reader check in run-os-sandboxed.