Require object values in serialization schemas

[OskarEichler]

Summary

• declare the function serialization payload as a JSON object
• declare include, extend, prepend, attribute, and alias members as JSON objects
• reject scalar, array, and null values that currently bypass properties and required

Reproduction

Draft-04 validation ignores properties and required for non-object instances unless an object type is declared. As a result, the six affected definitions currently accept values such as null, 1, "value", and [] even though their serialized forms are objects.

Verification

• existing schema suite: 11/11 passing
• external negative model: invalid primitive values are rejected and representative serialized objects remain accepted
• current primary plus stdlib suites: 4,140 tests / 66,231 assertions / 0 failures / 0 errors
• native build and complete signature validation
• all nine generated template outputs remain byte-identical
• Rust parser: 47 tests, formatting, and Clippy with warnings denied

No runtime or generated API behavior changes for valid serialized objects.