The shape helpers and pack/unpack tfuncs ran inside the kernel-inferred
path, where two failure modes produced confusing errors:

- A tfunc returning `nothing` on an indivisible width left the result
  untypable, surfacing downstream as `internal error: invalid terminators`.
- A `throw(ArgumentError(...))` in a shape helper became an unsupported
  `String` in kernel IR (`format_string`/`unsupported String` error),
  masking the intended message.

Make both layers total: pack/unpack tfuncs always return a concrete type
(via `fld`), and the shape helpers are pure arithmetic. Validation now
lives solely in the pack/unpack/reshape emit, which throws a clear
`IRError` (e.g. "unpack: 1 bytes do not evenly divide into Float32").
Valid reinterprets are unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>