Bound the create name and the folder child count at the facade

[FSM1]

Residual from the unknown-truncation slice

The truncation defect and the distinct dead-letter reason landed; two acceptance conditions from that issue live in crates/engine/src/facade.rs and were deliberately not taken while another slice owned the file:

• Bound the name a Command::Create accepts at the command boundary.
• Decide and enforce the folder child-count ceiling.

Acceptance

• An over-bound create name is refused at the boundary with a typed error, release-active.
• The folder child-count ceiling is decided, enforced, and its interaction with the truncation rule stated in one place.