schemagen: reject absent additionalProperties

Per the README contract, an object schema without an explicit
`additionalProperties` was documented as rejected at generation time,
but the code defaulted it to `false` and silently generated a strict
parser that rejects valid documents with extra properties.

Make the code match the documented contract by raising GenError when
`additionalProperties` is absent, with a clear message instructing the
author to set it explicitly to false. Update the README prose section
to list absent additionalProperties alongside true, and fix existing
schemas and tests to declare additionalProperties explicitly.

Closes #55

contrib/schemagen/weaseljson_schemagen.py

@@ -417,7 +417,13 @@ class Builder:
         tobj = TObj(name)
         if defname is not None:
             self._building[defname] = (tobj, nullable)
-        ap = node.get("additionalProperties", False)
+        ap = node.get("additionalProperties", None)
+        if ap is None:
+            raise GenError(
+                "additionalProperties is required for object schemas; set it "
+                "explicitly to false to reject unknown keys (absent "
+                "additionalProperties is not supported)"
+            )
         if ap is True:
             raise GenError("additionalProperties: true is not supported")
         if isinstance(ap, dict):