fix(setKey): use a sentinel to detect count auto-increment

[SAY-5]

The TypeScript conversion changed value === increment (function reference used as a sentinel) to value === increment() (which returns 1), so any later occurrence of a non-count argument with value 1 is incorrectly incremented onto the previous value (parser('-x 3 -x 1') returns { x: 4 } instead of { x: [3, 1] }). Restored the original semantics with a Symbol sentinel that flows from processValue to setKey.

Closes #506

[shadowspawn]

To be clear: is this PR entirely AI/LLM generated?

[SAY-5]

No. I traced the regression from #506 myself, confirmed the sentinel-reference behavior change in git history, and verified the Symbol fix against the failing case with the added test. Happy to walk through any part of the reasoning.