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),

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);

crates/clawsync-core/benches/cdc_bench.rs

@@ -58,8 +58,10 @@ fn cold_data(n: usize) -> Vec<u8> {
 // Benchmark
 // ─────────────────────────────────────────────────────────────────────────────
 
+type DataGen = fn(usize) -> Vec<u8>;
+
 fn bench_cdc(c: &mut Criterion) {
-    let datasets: &[(&str, fn(usize) -> Vec<u8>)] = &[
+    let datasets: &[(&str, DataGen)] = &[
         ("random", random_data),
         ("float", float_data),
         ("cold_only", cold_data),

crates/clawsync-core/src/simd_cdc.rs

@@ -151,6 +151,9 @@ pub fn chunk_scalar(data: &[u8], min: usize, avg: usize, max: usize) -> Vec<Chun
 /// Scalar processing for a single 16-byte (or shorter) slice, folding the
 /// updated hash back out.  Used by both SIMD paths when a hot byte is detected.
 #[inline(always)]
+// Hot SIMD inner-loop helper: each arg is a distinct cursor/limit; bundling
+// into a struct would add per-call overhead in the chunker fast path.
+#[allow(clippy::too_many_arguments)]
 fn scalar_window(
     data: &[u8],
     window: &[u8],
@@ -542,7 +545,7 @@ mod tests {
         let original = chunk_data_simd(&data);
         // Flip a hot byte in the middle to force a different boundary
         let mid = 150_000;
-        data[mid] = data[mid] & 0x3F; // ensure it's a hot byte (< 64)
+        data[mid] &= 0x3F; // ensure it's a hot byte (< 64)
         data[mid] ^= 0x11;
         let mutated = chunk_data_simd(&data);
         assert_ne!(original, mutated, "mutation should change chunk boundaries");