Flip cr benchmark run default to in-process after a parity run #484

Follow-up to #483 (A6). The in-process executor exists behind --in-process; the subprocess path remains the default.

Per the architecture report's incremental path:

1. Run a release-scale parity comparison — same suite, both modes, diff summary.jsonl (classification granularity, review.json provenance, manifest fields).
2. Flip the default to in-process; keep subprocess whenever --cr-bin is set (cross-version A/B stays a first-class feature).

Also queued from #483's final review: converge select.go on the same runtime source (shared profile/PR resolution, progress routing, taxonomy handling) while retaining pipeline.SelectionOnly.