refactor(codegen): extract SchemaFormatter behaviour and SchemaCore

Introduces two new modules to eliminate the ~780-line duplication between
ZodSchemaGenerator and ValibotSchemaGenerator:

- SchemaFormatter: behaviour defining ~20 output-syntax callbacks
  (wrap_optional, wrap_array, format_enum, format_string, etc.)
- SchemaCore: all shared logic delegating output to a formatter module
  (topological sort, dep discovery, field/action introspection,
   resource schema generation, regex safety helpers)

Author: directormac