Pass 12 milestone

Planned gate: MS-P12, defined by B14 in course/design/B14-multimodal.md. The pass composes MS-L13, MS-L14, MS-mm-corpus, MS-L15, and MS-multimodal-prod, then reruns the smoke steps of MS-P0 through MS-P11.

This path remains a design reading stage until Step C merges B14 and its modules, milestones, and chapter files.