feat: add offline project compile regression suites

[meaningforger]

Summary

• Implement RFC-0090 phase 1 as metis project test --mode compile using the project source loader and production CompileService, without database access or secret resolution.
• Add strict versioned YAML/JSON suites for exact output-schema assertions, optional warning and typed SQL/parameter snapshots, and stable semantic-error expectations.
• Emit private atomic JSON reports with pass/fail/not-run status, bounded diagnostics, digests, and CI exit codes; add a demo suite and current contract documentation.

Runtime result suites, policy-denial assertions, and JUnit output remain future phases of RFC-0090.

Validation

• make check
• CGO_ENABLED=0 go test ./app/tooling/regression ./cmd/metis
• CGO_ENABLED=1 go test -tags duckdb ./app/tooling/regression ./cmd/metis
• Demo compile suite passed through the CLI.