spec: document subpackage artifact tradeoff

Explain why subpackages share the root artifact model while rejecting the sole unsupported strip control at validation time.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

Author: cpuguy83

subpackage.go

@@ -130,6 +130,9 @@ func (s *SubPackage) validate() error {
 	}
 
 	if s.Artifacts != nil {
+		// Subpackages deliberately reuse the root Artifacts public model. DisableStrip
+		// is currently its sole unsupported field, so accepting one representable
+		// invalid state avoids maintaining nearly identical public types.
 		if s.Artifacts.DisableStrip {
 			errs = append(errs, fmt.Errorf("artifacts: disable_strip is only valid for root package artifacts"))
 		}