proposal: preserve `go/types.Var.Kind` semantics for receiver and result vars on Go 1.25+

Background

Go 1.25 added semantic categories for variables in go/types.

• go/types.Var.Kind
• go/types.Var.SetKind
• go/types.VarKind

In upstream Go type checking, method receivers are created as RecvVar.

• go/types/signature.go creates receiver with RecvVar
• go/types/object.go documents NewParam and SetKind
• go/types test expects recv: RecvVar

Current behavior in XGo

In XGo compile paths, receiver and some result-like vars are created through NewParam(...) and are not relabeled with SetKind(...).

• cl/func_type_and_var.go: receiver built via ctx.pkg.NewParam(...)
• cl/compile.go: synthetic this built via types.NewParam(...)

This makes receiver vars indistinguishable from ordinary parameters through Var.Kind.

Proposal

For Go 1.25+ builds in XGo:

• Set receiver vars to types.RecvVar immediately after creation
• Set result vars to types.ResultVar where applicable
• Keep ordinary parameters as types.ParamVar

No source-position or naming behavior change is required.

Acceptance criteria

Given func (r *T) M(x int) (y int) {}:

• r.Kind() == types.RecvVar
• x.Kind() == types.ParamVar
• y.Kind() == types.ResultVar

Also:

• Existing tests continue to pass.
• Add focused tests for receiver, parameter, and result kind classification.