feat(agc): DMA_DATA cache policy hints and GDS source/destination
- **Cache policy:** the source (bits 13–14) and destination (bits 25–26) cache-policy fields of `DMA_DATA` dword 1 are accepted. They're cache hints with no effect on the copied data. The remaining reserved bits are still rejected.
- **GDS storage:** GDS is modelled as one 64 KiB, zero-initialised store shared by the whole device, behind a mutex.
- **GDS source/destination:** select `1` reads from or writes to that store:
- the validator requires the high address dword to be 0 and the range to fit inside the 64 KiB;
- `Execute` copies between memory, immediate data and GDS in any combination;
- `ResolveStore` returns nothing for a GDS destination, since there's no guest memory to write, and for a GDS source it resolves the bytes from the store.
- Register and prefetch destinations are still rejected.
- Nothing is faked for shaders: a shader that uses GDS still fails validation (unsupported descriptor role).
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