Why does Box64 use a fixed 128 bitmap for small allocations?
In `src/custommem.c` the function:
```c
void* map128_customMalloc(size_t size, int is32bits) {
    size = 128;
    …
}

```
hard-codes every allocation to 128 bytes. 
However, In my profiling (box64 + wine just like #2511 )
most allocation requests are far smaller (e.g. ≤ 56 bytes), so this can increase internal fragmentation.

![Image](https://github.com/user-attachments/assets/1cab3010-826f-4204-983a-09e8d6aeea7b)


4 | 1030
-- | --
8 | 85
16 | 716
32 | 268
40 | 14
56 | 7998
64 | 34
96 | 81
120 | 1152
128 | 24



#### Questions: 
1. What was the original design rationale for fixing the block size at 128?

2. Is it for alignment, page-size granularity, mmap overhead, or something else?

3. Would it be beneficial to reduce this to 64?

I’m happy to draft a PR to change the default to 64 if that would help.

### Environment
1. Box64 version: Box64 riscv64 v0.3.7 926a5980 with Dynarec built on Jun 13 2025
2. Build flags:
  ```
cmake -G Ninja \
  -DBOX32=ON \
  -DRV64=1 \
  -DRV64_DYNAREC=ON \
  -DGDBJIT=ON \
  -DCMAKE_BUILD_TYPE=RelWithDebInfo \
  -DCMAKE_C_COMPILER=gcc \
  ..
```
3. Platform: RISC-V 64, Debian (chroot)
4. CPU: 4× cores(Andes AX45MP), Little Endian
5. page size 4096
6. Kernel: 6.1.47+
7. gcc :(Debian 14.2.0-19) 14.2.0
8. glibc :(Debian GLIBC 2.41-7) 2.41

Thanks!