Part of #188474.

DeviceBuffer can only bind as a uniform today. Compute and storage workflows need a storage-buffer binding, and Shader needs to expose storage slots reflected from the bundle.

Changes

• Add a storage-buffer binding path to DeviceBuffer and the native glue in lib/gpu.
• Surface storage-buffer reflection so a Shader exposes storage slots, reading the bundle fields from [Flutter GPU] shader bundle carries compute workgroup size and storage buffer bindings #188477.

Testing

Bind a storage buffer in a render shader and read it back, the simplest path that needs no compute.

Dependencies

#188477.