Claude review: gpu: nova-core: gsp: add tests for continuation records

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Good test coverage using kunit_tests. The tests:

1. Use a non-zero-sized `TestHeader` (64 bytes) to exercise the `MAX_FIRST_PAYLOAD` calculation realistically.
2. Generate a non-trivial test pattern (`i ^ (i >> 8)`) that doesn't repeat every 256 bytes.
3. Cover boundary conditions precisely:
   - 0 payload → 0 continuations
   - `MAX_FIRST_PAYLOAD` → 0 continuations (exact fit)
   - `MAX_FIRST_PAYLOAD + 1` → 1 continuation (just over)
   - `MAX_FIRST_PAYLOAD + MAX_CMD_SIZE` → 1 continuation (exact 2nd chunk)
   - `MAX_FIRST_PAYLOAD + MAX_CMD_SIZE + 1` → 2 continuations (just over 2 chunks)
   - `MAX_FIRST_PAYLOAD + 3.5 * MAX_CMD_SIZE` → 4 continuations
4. Verify both the number of continuations AND that the reassembled data matches the original pattern.

The `read_payload` helper correctly creates an SBufferIter with an empty second slice (`&mut []`), which works because each individual chunk (split command payload or continuation record) fits within a single buffer.

**Minor observation:** `TestHeader` manually implements `FromBytes` and `AsBytes` with `// SAFETY` comments. This is correct for `[u8; 64]`, but if the `derive` macros from zerocopy/kernel::transmute were available for this type, it would be cleaner. However, this is likely a constraint of the kernel's transmute API and is fine as-is.

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