Fix counter panic on a uint or float field with an explicit --flag=N

[chuenchen309]

Problem

A counter flag with an explicit value (--flag=N, the documented long form) panics when the field is a uint or float:

var cli struct {
    Count uint `type:"counter"`
}
kong.Parse(&cli, "--count=3")
// panic: reflect: call of reflect.Value.SetInt on uint Value

The README says counter works on a "numeric field" and "Can accept -s, --long or --long=N", and the plain increment form (-ccc) already works on uint/float fields — only the explicit-value form crashes.

Cause

In counterMapper, the increment path switches on Int/Uint/Float kinds, but the explicit-value path assigned unconditionally:

case string:
    n, err := strconv.ParseInt(v, 10, 64)
    ...
    target.SetInt(n)            // panics on a uint/float target
case int, ...:
    target.Set(reflect.ValueOf(v))   // panics on a kind mismatch (e.g. int value, uint target)

SetInt is only valid on int kinds, so a uint/float64 counter panics. The resolver-supplied numeric branch had the same kind-mismatch issue.

Fix

Parse the explicit value to an int64, then assign by the target's kind (SetInt/SetUint/SetFloat), mirroring the increment path just below it. Extended TestCounter with the previously-missing --uint=N / --float=N cases (it only covered --int=5 explicitly).

go test ./... passes; golangci-lint/gofmt clean.

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Disclosure: authored by an AI coding agent (Claude Code) running on this account — it found the bug, wrote the repro/test, the fix and this description. I review every change and am accountable for it; the verification above is real and re-runnable from the diff. Happy to adjust anything.