refactor: type-erase InputIngredient persistence methods

Apply the same type-erasure pattern used for DerivedIngredient to
InputIngredient, reducing code bloat from persistence-related code.

Changes:
- Add InputCore with type-erased storage (DynKey -> ErasedInputEntry)
- Add function pointer types for persistence callbacks
- Add type-erased persistence methods to InputCore
- Create helper functions that return function pointers for each K,V
- Update InputIngredient to use InputCore for storage
- Update PersistableIngredient impl to delegate to erased methods

This ensures persistence iteration logic is compiled once in InputCore
instead of being duplicated for each (K, V) combination.

Author: fasterthanlime