fix(vrm): normalize VRMA humanoid rotations

[doodlewind]

Summary

• convert raw VRMA glTF bone rotations into VRM normalized humanoid space before retargeting
• convert normalized VRM 1 poses into each VRM 0 model bone raw local rest basis
• match Three.js world-matrix decomposition, including non-uniform-scale shear
• add synthetic basis/shear coverage plus optional Persona/VRoid fixture goldens

Why

The previous loader copied raw local VRMA quaternions directly (apart from the VRM 1 to VRM 0 facing flip). Raw rest axes differ between rigs, so wrists, hands, ankles, and feet were visibly twisted even though humanoid bone names matched.

This is a generic pocket-vrm correction with no Persona-specific runtime branch. The Persona fixture is optional, git-ignored test data used only as an independent three-vrm golden.

Validation

• cargo test --manifest-path engine/Cargo.toml -p pocket-vrm (19 unit + 6 fixture tests)
• cargo clippy --manifest-path engine/Cargo.toml -p pocket-vrm --all-targets -- -D warnings
• rustfmt --edition 2024 --check engine/crates/pocket-vrm/src/vrma.rs engine/crates/pocket-vrm/tests/fixtures.rs
• git diff --check
• visually checked latest Persona packaged idle and multiple speaking frames against the current Persona renderer; limb axes are natural after retargeting

Stack

This draft currently targets creative-education only because it is built on the generic window visibility commit in #204. The VRMA change is independent and should be retargeted to main after #204 lands.