Shared memory support for GPU scratchpad

[tetsuo-cpp]

Summary

Add support for GPU shared memory (__shared__ in CUDA) to enable tiled algorithms.

Motivation

Tiled matmul is the baseline practical GPU implementation. Without shared memory, you're limited to naive matmul which is ~100x slower. Shared memory acts as a programmer-managed cache shared across all threads in a block.

Design considerations

• Need a way to declare shared memory (size, element type)
• Need a way to read from and write to shared memory
• Possible syntax: SHARED <name> <size> declaration + reuse @ !
• MLIR mapping: memref.alloc in GPU shared address space, or gpu.alloc with appropriate memory space

Implementation notes

• May require extending !forth.stack or adding a new shared memory type
• The memref should be in GPU shared address space (address space 3 for NVVM)
• Allocation should be at kernel entry, not per-thread

Priority

High — required for any practical tiled GPU algorithm.