internal/pkg: expose extra methods to file

This is useful for FileArtifact processing another stream.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg.go

@@ -196,14 +196,7 @@ func (f *FContext) GetArtifact(a Artifact) (
 //
 // Methods of RContext are safe for concurrent use. RContext is valid
 // until [FileArtifact.Cure] returns.
-type RContext struct {
-	// Address of underlying [Cache], should be zeroed or made unusable after
-	// [FileArtifact.Cure] returns and must not be exposed directly.
-	cache *Cache
-}
-
-// Unwrap returns the underlying [context.Context].
-func (r *RContext) Unwrap() context.Context { return r.cache.ctx }
+type RContext struct{ common }
 
 // An Artifact is a read-only reference to a piece of data that may be created
 // deterministically but might not currently be available in memory or on the
@@ -959,7 +952,7 @@ func (c *Cache) openFile(f FileArtifact) (r io.ReadCloser, err error) {
 				}
 			}()
 		}
-		return f.Cure(&RContext{c})
+		return f.Cure(&RContext{common{c}})
 	}
 	return
 }
@@ -1455,7 +1448,7 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 		if err = c.enterCure(a, curesExempt); err != nil {
 			return
 		}
-		r, err = f.Cure(&RContext{c})
+		r, err = f.Cure(&RContext{common{c}})
 		if err == nil {
 			if checksumPathname == nil || c.IsStrict() {
 				h := sha512.New384()