feat[gpu]: support mixed-width types in dynamic dispatch (#7331)

Add per-stage PType tracking so input stages (e.g. u8 dict codes) can
differ from the output type. The kernel widens narrow loads to the
output type via load_element() instead of requiring a separate widening
pass.

Kernel changes:
- Byte-addressed shared memory (smem offsets are now in bytes, not
elements) so stages with different element widths can coexist
- PTypeTag per source op drives load_element() for type-widening loads
- Fuse scalar_ops into the source_op loop for non-BITUNPACK input stages

Plan builder (Rust):
- Track smem_byte_offset per stage instead of element offset
- Emit PTypeTag alongside each source op in the packed plan
- Plumb per-stage and per-op elem_bytes through the C ABI

```
100M benchmark results (GH200 480GB, SM 9.0) - % ref to reduction in duration here:
  for_bitpacked_6bw      170.75 µs  2181.7 GiB/s  -2.5%
  dict_256vals_bp8bw     191.90 µs  1941.3 GiB/s  -7.4%
  runend_100runs         145.63 µs  2558.1 GiB/s  -21.0%
  dict_64vals_nested     176.61 µs  2109.3 GiB/s  -6.2%
  alp_for_bp_6bw_f32    179.23 µs  2078.5 GiB/s  -4.6%
```

---------

Signed-off-by: Alexander Droste <alexander.droste@protonmail.com>

Author: 0ax1