Workgroup interface materialization

This subsystem owns the Workgroup-facing portion of interface construction:

• types.rs builds metadata-shaped Workgroup backing types and isolates Workgroup type graphs from explicit StorageBuffer layout decoration.
• pointers.rs materializes rooted pointer uses after entry parameters are spliced to real interface variables. The generic rooted-pointer machinery stays here because Workgroup and StorageBuffer/Private roots require the same transitive type repair.
• zero_initialize.rs inserts the kernel-entry zero-fill and Workgroup memory barrier that make the validation harness's refinement of undefined threadgroup contents deterministic.

passes/stage_input decides which decoded AIR parameter is Workgroup memory and invokes interface materialization in the established stage-input order.