fix: count duplicate keys independently of prototype names

[fhgffy]

With bracketedArray: false, repeated keys such as constructor and toString currently lose all but their last value. The duplicate counter inherits those names from Object.prototype, so its first increment starts from a function instead of zero.

Use a null-prototype counter, matching the parser's output objects. Tests cover three repeated values and scalar/default-mode controls for four prototype member names. This is separate from #299, which changes duplicate tracking across sections.

Validation: the new tests fail on the unchanged base; npm test passes all five test files with 100% coverage, ESLint, and template checks on Node 24.15.0.