Implement `harmonize_fct()`

[jonthegeek]

This function will map an input vector to a factor, optionally applying a lookup table. It is the core vector-level engine for harmonizing categorical data.

The signature should be:

harmonize_fct(.data, ..., .spec = NULL, lookup = NULL)

MVP Behavior:

• The function will take an input vector (.data).
• It will apply the named vector provided in lookup to translate values.
• If a .spec object (from specify_fct()) is provided, it will validate the resulting values against the levels in the spec, coercing the final output to a factor.
• If .spec is NULL, the function will perform the mapping but skip the validation, returning a factor.