Improve hash mixing to eliminate bitvector-expression hash-table clustering (#10120)

Large Python bitvector workloads were hitting a sharp performance cliff
during `Solver.add(...)`, consistent with severe hash-table clustering
in expression-heavy assertion paths. The issue was sensitive to input
size/alignment, indicating weak low-bit dispersion in hash combination.

- **Hash mixing update (`src/util/hash.h`)**
- Replaced the old `combine_hash(h1, h2)` arithmetic/xor sequence with
stronger mixing:
    - boost-style combine step
    - `hash_u(...)` finalization
- Goal: improve low-bit entropy used by chained hash-table bucket
selection under aligned/high-volume AST patterns.

- **Regression guard and A/B comparison (`src/test/chashtable.cpp`)**
- Added `tst_combine_hash_low_bits()` and invoked it from
`tst_chashtable()`.
- The test stresses aligned first components (`i << 12`) combined with a
fixed seed.
- Added an in-test comparison between the **old** and **new** pairwise
hash combiners and validates:
- reduced collision counts for low-bit projections (8-bit and 16-bit
suffixes),
    - improved low-bit uniformity for 8-bit and 16-bit suffixes,
- reported prefix/suffix uniformity metrics (high/low 8 and 16 bits) for
visibility in test output.

```cpp
static inline unsigned combine_hash(unsigned h1, unsigned h2) {
    h1 ^= h2 + 0x9e3779b9 + (h1 << 6) + (h1 >> 2);
    return hash_u(h1);
}
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/util/hash.h

@@ -57,10 +57,8 @@ static inline unsigned hash_ull(unsigned long long a) {
 }
 
 static inline unsigned combine_hash(unsigned h1, unsigned h2) {
-    h2 -= h1; h2 ^= (h1 << 8);
-    h1 -= h2; h2 ^= (h1 << 16);
-    h2 -= h1; h2 ^= (h1 << 10);
-    return h2;
+    h1 ^= h2 + 0x9e3779b9 + (h1 << 6) + (h1 >> 2);
+    return hash_u(h1);
 }
 
 static inline unsigned hash_u_u(unsigned a, unsigned b) {
@@ -256,4 +254,3 @@ static inline unsigned mk_mix(unsigned a, unsigned b, unsigned c) {
     return c;
 }
 
-