feat: Add compound shape and convex decomposition support

[hsheth2]

Closes #44 and supersedes #303.

• Added support for compound shapes and convex decomposition to the JavaScript bindings
• The API uses separate arrays for shapes/positions/rotations (matches existing joint API pattern, unlike wip untested compound shapes #303 which created an Isometry class)
• Added two testbed3d demos showcasing the features, and tested these manually in my browser.

Known limitation: compound shapes cannot be fully deserialized from snapshots (the Rust/WASM layer doesn't expose sub-shape query APIs). This only affects save/load workflows that need to inspect compound internals - normal creation and physics simulation work fine.

Disclaimer: I used Claude Code to produce this PR, but have manually reviewed the code myself already.