meta(testing): close all P0 behavior rows from deferred/implemented to tested

### Summary
Drive P0 behavior matrix to fully tested status.

### Why
`v1-mvp.md` completion requires passing tests for P0 behaviors.

### Scope
- Turn B-001..B-009 into concrete tests
- Update matrix statuses with evidence
- Track residual risk for untested edges

### Acceptance Criteria
- All P0 rows marked `tested`
- CI runs and passes all P0 tests
- Matrix references test IDs/files