Session 2B-ii mandatory-checkpoint rehearsal -- exercises every persistent_projects.py function (including the Gap 1 ownership fix) against REHEARSAL_DATABASE_URL, NEVER the real DATABASE_URL. Refuses to run at all if REHEARSAL_DATABASE_URL is unset, rather than silently falling back to production. This process runs on the real Railway service (inside its private network) specifically because railway run cannot tunnel *.railway.internal DNS to a local machine -- see persistent_projects.py's own docstring on _DATABASE_URL_OVERRIDE.

Returns only pass/fail booleans and small derived facts (counts, hashes, booleans) -- never a connection string, never full house_json content, never a raw house_ref/project_id beyond what's needed to show one check succeeded.