refactor(remote): drop ReferrerCapability wrapper for atomic state field

Remove referrers_state.go and its exported ReferrerCapability /
ReferrerMergePool types in favor of the leaner upstream shape: a plain
atomic int32 referrersState field and an inline
syncutil.Pool[syncutil.Merge[referrerChange]] merge pool. Keeps the
unexported referrersState iota (unknown/supported/unsupported) as the
capability primitive and preserves the void, first-wins
SetReferrersCapability. ErrReferrersCapabilityAlreadySet is retained to
match the v2 public surface.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Terry Howe <terrylhowe@gmail.com>

Author: TerryHowe

registry/remote/builder_test.go

@@ -614,7 +614,7 @@ func TestNewRepositoryWithProperties_ReferrersAPISupported(t *testing.T) {
 
 	// Conflicting set is silently ignored; capability should remain supported.
 	repo.SetReferrersCapability(false)
-	if !repo.getReferrersCapability().IsSupported() {
+	if repo.loadReferrersState() != referrersStateSupported {
 		t.Error("conflicting SetReferrersCapability(false) should be ignored when already set to supported")
 	}
 }
@@ -637,7 +637,7 @@ func TestNewRepositoryWithProperties_ReferrersAPIUnsupported(t *testing.T) {
 
 	// Conflicting set is silently ignored; capability should remain unsupported.
 	repo.SetReferrersCapability(true)
-	if !repo.getReferrersCapability().IsUnsupported() {
+	if repo.loadReferrersState() != referrersStateUnsupported {
 		t.Error("conflicting SetReferrersCapability(true) should be ignored when already set to unsupported")
 	}
 }

registry/remote/referrers.go

@@ -60,9 +60,9 @@ type referrerChange struct {
 }
 
 var (
-	// ErrReferrersCapabilityAlreadySet is returned by ReferrerCapability.SetSupported()
-	// and ReferrerCapability.SetUnsupported() when the capability has already been set
-	// to a conflicting value.
+	// ErrReferrersCapabilityAlreadySet is reserved to signal that the referrers
+	// capability of a repository has already been set to a conflicting value.
+	// The capability is fixed once set; the first value wins.
 	ErrReferrersCapabilityAlreadySet = errors.New("referrers capability cannot be changed once set")
 
 	// errNoReferrerUpdate is returned by applyReferrerChanges() when there