gpu_replay_buffer.rs had three sites that cast `&CudaSlice<i32>` to
`&CudaSlice<u32>` via raw-pointer transmutes to satisfy kernel
signatures (gather_u32, scatter_insert_u32). This changes the element
type of a `&mut` reference through `as *mut`, which is UB under Rust's
strict aliasing model even though i32 and u32 share a memory layout.
Episode-id values are always non-negative (counters of the form
`(write_cursor + j) % capacity`), so u32 is the correct storage type.
Changed:
- 3 field types: episode_ids, sample_episode_ids, insert_ep_buf
(CudaSlice<i32> → CudaSlice<u32>)
- Allocators: a32i → a32u at 4 sites
- Vec<i32> → Vec<u32> at ep_ids_host, with `as u32` cast
- Deleted 3 raw-ptr transmute blocks (lines 491-492, 689-690)
- Public sample_episode_ids_ref() return type i32 → u32
- Deleted now-unused a32i helper (fn never called after the change)
Per BORROW learned pattern option (3): "change the declared type".
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