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

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

Patch Review

Comprehensive kunit tests covering:
- Zero payload (no split)
- Payload exactly at `MAX_FIRST_PAYLOAD` (fits in one command)
- Payload at `MAX_FIRST_PAYLOAD + 1` (triggers 1 continuation)
- Payload at `MAX_FIRST_PAYLOAD + MAX_CMD_SIZE` (fills exactly 1 continuation)
- Payload at `MAX_FIRST_PAYLOAD + MAX_CMD_SIZE + 1` (triggers 2 continuations)
- Larger payload with fractional continuation fill

The test verifies both the number of continuation records and that the concatenated data matches the original pattern. The `TestHeader` uses a 64-byte struct to exercise the non-trivial case where the first command has less payload space due to the header.

The `TestPayload::generate_pattern` uses `(i ^ (i >> 8)) as u8` to avoid a repeating 256-byte pattern, which is a nice touch for catching off-by-one errors in splitting.

One very minor nit: `read_payload` creates an `SBufferIter` with `[buf.as_mut_slice(), &mut []]` — the empty second slice is fine but could have a brief comment. Not a blocking issue.

No issues. Good test coverage.

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