Feat: Allow Pre-save hooks on the create operations

[VoidClancy]

1- Add beforeCreate hook to CreateMany and CreateManyAndReturn wrappers to allow mutation on the struct before it's serialized to SQL rows

2- Add ToRowMap() on {Model}Create as the single exit point from the struct
representation, with per field nil checks, default injection, and CUID/UUID generation (if present)

3- establish a flow of create api takes the FieldAssignment spread and generates a struct via assignmentsTo{Model}Create, and the pre-save hook mutates that struct if preset, then, the base struct is converted to to a map via ToRowMap() that's used to generate the SQL rows in create operations

4- Remove RecordsToRowMaps generator function because ToRowMap() now owns the
struct to map conversion with proper defaults, eliminating the duplicated
default/CUID logic between RecordsToRowMaps and the single-create path

5- Remove Assignments() on {Model}Create because ToRowMap() replaces it as the single back-conversion path structs no longer round-trip through Field assignment

6- Remove validateFn, rowMapFn, and singleCreateFn params from executeCreateMany/AndReturn
since validation is handled at FieldAssignment level in the wrapper and map conversion is inline via ToRowMap(), the fallback per-row path uses mapToColsVals + executeInsert instead of re-entering singleCreateFn (which would double-fire hooks, and it did)