refactor: clippy fixes (assign_op, type_complexity, unnecessary max)

Resolve -D warnings under Rust 1.95 clippy:

- simd_cdc.rs: use compound assignment in unit test; document and
  scope-allow too_many_arguments on the SIMD inner-loop helper
  (each parameter is a distinct cursor/limit on the hot path).
- merkle.rs: drop `(usize).max(0)` which is always \u22650.
- benches: factor `fn(usize) -> Vec<u8>` into a `DataGen` type alias
  to satisfy clippy::type_complexity.

No behavioral changes; all workspace tests pass.

crates/clawhdf5-onion/src/merkle.rs

@@ -540,7 +540,7 @@ mod tests {
             let all_entries: Vec<(u64, [u8; 32])> = (0..total as u64)
                 .map(|r| (r, rev_hash(r)))
                 .collect();
-            let keep_count = ((total as f64 * keep_frac) as usize).max(0);
+            let keep_count = (total as f64 * keep_frac) as usize;
             let remote_entries = &all_entries[..keep_count];
 
             let local  = RevisionMerkleTree::build(&all_entries);