fix: allow reflect.StructOf dynamic struct fields in expr.Compile

[Copilot]

reflect.StructOf requires PkgPath to be set for lowercase-named fields, which causes Go's reflection API to treat them as unexported. PR #846's IsExported() check in the checker then silently skips these fields, producing an "unknown name" compile error for what is a valid runtime-accessible field.

Changes

• checker/nature/utils.go: Scope the IsExported() guard to named struct types only (s.rType.PkgPath() != ""). Anonymous types (including all reflect.StructOf outputs, whose type-level PkgPath() is always "") allow all fields through. The Checker not failing when accessing unexported struct fields #844 fix for regular Go structs is fully preserved.

• vm/runtime/runtime.go: Add exported ValueInterface(reflect.Value) any that falls back to type-specific getters (Bool(), Int(), String(), etc.) when Interface() would panic on an unexported field. Update FetchField and Fetch to use it; extend the IsExported gate in Fetch to also pass anonymous-struct fields.

• builtin/lib.go: Mirror the same gate change and switch to runtime.ValueInterface in the get function.

Example

dynType := reflect.StructOf([]reflect.StructField{
    {Name: "value", Type: reflect.TypeFor[bool](), PkgPath: "github.com/some/package"},
})
env := reflect.New(dynType).Elem().Interface()

// Was: "unknown name value" in v1.17.7+
// Now: compiles and evaluates correctly
program, err := expr.Compile("value", expr.Env(env))
result, err := expr.Run(program, env) // → false (zero value)

ValueInterface covers all primitive kinds; non-primitive unexported fields in anonymous structs (e.g. nested struct, slice) will still panic with a descriptive message, as there is no way to retrieve them without unsafe.

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