[hiracchi]

What

Adds architecture 8.13 "Motion Files (planned)" (EN/JA): a YAML file (yui-motion/1) that scripts camera turns about any axis, zoom, wait, frame stepping and representation changes (set: visible / opacity, with fade) for repeatable movies.

Why

Users want to turn about a chosen axis by a chosen angle, and to save a fixed motion as a movie. The design keeps representations in the workspace (no second definition), validates the whole file before the first frame, and moves the camera rotation to quaternions (no elevation clamp) without changing the saved CameraState.

Design only; nothing is implemented yet. Planned PR order: (1) quaternion rotation, (2) file reading and export, (3) set.

Checks (scripts/check.sh --trunk)

1. cargo check (native): PASS
2. cargo check (wasm32): PASS
3. cargo test: PASS
4. cargo fmt: PASS
5. cargo clippy (native): PASS
6. cargo clippy (wasm32): PASS
7. trunk build: PASS
8. EN/JA doc sync (architecture 73 = 73): PASS

Not checked: nothing skipped. No CHANGELOG entry, because there is no user-visible change.