data_loader/rng: shuffleSeed is uint64_t but rngSetSeed takes uint32_t — silent truncation

Found during the #381 review (pre-existing, untouched by the quick-wins PR):

`dataLoader_t.shuffleSeed` is declared `uint64_t` (DataLoader.h), but `initDataLoader` passes it to `rngSetSeed`, which takes `uint32_t` — the upper 32 bits are silently discarded. Two seeds differing only in the high word produce identical shuffles.

Fix options: (a) narrow the field to `uint32_t` (breaking, honest — matches the global XorShift32 state width and the Python mirror), or (b) fold the high word in (e.g. xor) before seeding. Option (a) preferred: the RNG is 32-bit by design and byte-mirrored in Python; a 64-bit seed surface is a false promise.

Regression test: two loaders seeded `0x1` and `0x1_0000_0001` must produce different permutations (currently identical).

Read DataLoader.h, initDataLoader, and rngSetSeed first, then inspect the Python mirror mentioned in the issue to confirm the intended 32-bit seed surface. Add regression coverage for loaders seeded with 0x1 and 0x1_0000_0001, and reconcile that expected behavior with the issue’s preferred narrowing option before marking the work done.

Written by the indexing model from the issue text.