fix(bls): size pairing buffers for 32-bit targets (#531)

## Summary

- compute the 384-bit BLST field size independently of pointer width
- keep pointer-width sizing only for the `DST` and `DST_len` fields

## Why

`Pairing.sizeOf()` divided a 384-bit field width by `@sizeOf(usize)`.
This happened to produce the correct 48-byte field size on 64-bit
targets, but produced 96 bytes on 32-bit targets. BLST represents the
field as either six 64-bit limbs or twelve 32-bit limbs, so its size
remains 48 bytes.

The old calculation therefore reserved a 6352-byte pairing context on
32-bit targets instead of BLST's 3184 bytes. This was safe but doubled
every pairing buffer, including the worker pool's buffer array.

This was identified while discussing 32-bit capacity handling in
#522 (comment).

Author: GrapeBaBa

src/bls/Pairing.zig

@@ -26,7 +26,7 @@ pub fn init(buffer: *align(buf_align) [Self.sizeOf()]u8, hash_or_encode: bool, d
 ///
 /// This is safe because blst is statically linked to this binding.
 pub fn sizeOf() usize {
-    const vec384_size = 384 / @sizeOf(usize);
+    const vec384_size = 384 / std.mem.byte_size_in_bits;
     const vec384fp12_size = vec384_size * 12;
 
     const point_e1_affine_size = vec384_size * 2;