schemagen: drop support for additionalProperties: true

Following review feedback, the generator no longer supports permissive
objects. Changes:

- Reject `additionalProperties: true` at generation time.
- Treat an absent `additionalProperties` as `false`, so every object is
  strict by default and unknown keys are rejected during parsing.
- Remove the now-dead permissive-object infrastructure: `Kind::Skip`,
  `Cat::Skip`, `kSkip`, the per-frame `unknown` key set, and `isStrict()`.
- Update the README feature/rejection tables accordingly.
- Remove the permissive "loose" object from example.schema.json and the
  associated tests from test_gen.cpp.
- Add Python unit tests verifying the new `additionalProperties` behavior.

All tests pass (`ctest --output-on-failure`).

contrib/schemagen/README.md

@@ -45,7 +45,7 @@ into the result, so it is non-movable.
 | `$ref` to `$defs`/`definitions`               | the referenced named struct            |
 | recursive `$ref`                              | `std::unique_ptr<T>` (cycle broken)    |
 | `additionalProperties: false`                 | unknown keys rejected                  |
-| `additionalProperties` absent / `true`        | unknown keys' values skipped           |
+| `additionalProperties` absent / `true`        | not supported (rejected at generation) |
 
 ## Schema violations (rejected at parse time)
 
@@ -55,15 +55,15 @@ into the result, so it is non-movable.
 - value not in a string `enum`
 - a number not representable in the target type (e.g. `1.5` for an `integer`)
 - duplicate object keys
-- unknown key under `additionalProperties: false`
+- unknown key in any object
 
 ## Not supported (rejected at generation time, no fallback)
 
 `oneOf` / `anyOf` / `allOf` / `not` / `if`-`then`-`else`,
 `patternProperties`, `additionalProperties` with a schema (typed map),
-`prefixItems` (tuples), `const`, `dependentSchemas`/`dependentRequired`,
-union `type` lists other than `["T", "null"]`, non-string enums, and remote
-(`$ref` to other documents).
+`additionalProperties: true`, `prefixItems` (tuples), `const`,
+`dependentSchemas`/`dependentRequired`, union `type` lists other than
+`["T", "null"]`, non-string enums, and remote (`$ref` to other documents).
 
 ## Notes