feat: emit typed wrapper for primitive oneOf schemas

Detects schemas where every oneOf variant is a JSON primitive (string,
number, integer, boolean, null) and emits a wrapper Go type instead of
falling through to interface{}. The wrapper holds the decoded value in
a private field and exposes:

  UnmarshalJSON   - dispatch on json.Decoder.Token kind so a number
                    that also matches integer cannot ambiguously
                    satisfy two variants.
  MarshalJSON     - emit the underlying value or null.
  UnmarshalYAML
  MarshalYAML
  Value()         - typed getter returning any.
  IsZero()        - supports omitzero.
  As<Kind>()      - typed accessor per declared variant
                    (AsString / AsNumber / AsBool / IsNull).

Detection wires into both generateDeclaredType (root) and
generateTypeInline (object property) so the wrapper is emitted whether
the oneOf is the root schema or a nested field.

Includes golden-file fixtures under tests/data/oneOfPrimitive/ covering
number+string, number+string+bool, string+null, and an in-field case
that mirrors the original motivating use case (a "value" property
typed as number|string|bool).

Author: plheide

README.md

@@ -163,7 +163,7 @@ only specific validations remain to be fully implemented.
   * [ ] Boolean subschemas (§6.7)
     * [ ] `allOf`
     * [ ] `anyOf`
-    * [ ] `oneOf`
+    * [x] `oneOf` (primitive variants only; **note**: object discriminated unions pending)
     * [ ] `not`
   * [ ] Semantic formats (§7.3)
     * [x] Dates and times