core: fix odd-bit primitive edge cases
Mask the partial storage byte when zero-extending odd-bit APInt
values, and make reinterpret padding checks inspect isbits union
members so odd-width primitives cannot bypass the guard.
Add focused coverage for runtime odd-width zero extension and
reinterpret rejection through an isbits union field.
Co-authored-by: Codex <codex@openai.com>
