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/benches/tdt_bench.rs

@@ -73,7 +73,8 @@ fn bench_compress(data: &[u8], codec: Codec) -> usize {
 fn bench_tdt_compress(c: &mut Criterion) {
     let sizes = [4096usize, 65536];
 
-    let datasets: &[(&str, fn(usize) -> Vec<u8>, usize)] = &[
+    type DataGen = fn(usize) -> Vec<u8>;
+    let datasets: &[(&str, DataGen, usize)] = &[
         ("f32_smooth", make_f32_smooth, 4),
         ("f32_random", make_f32_random, 4),
         ("int32_random", make_int32_random, 4),