Simplify the Stop transition contract

[hamzamerzic]

Summary

• rename the mounted glyph stack to match its actual role and correct stale transition comments
• make one CSS timing value own both halves of the sequential Stop handoff
• remove obsolete implementation guards and over-specific assertions while retaining the behavior contracts

This is a maintainability follow-up to #539. It preserves the rendered timing and interaction exactly while reducing duplicated knowledge and brittle test surface.

Testing

• focused composer interaction tests
• production/PWA build
• rendered timing verification

Co-authored-by: Möbius Agent mobius-agent@users.noreply.github.com