test: exhaustive unit tests for all provider-specific interfaces

Add 740 total tests (up from ~200) covering:
- Compile-time interface compliance for all providers
- GetCapabilities assertions for every provider
- Parse function edge cases: empty, malformed, single-entry, multi-entry
- apt: extract inline parse logic into testable functions (parsePolicyCandidate,
  parseUpgradeSimulation, parseHoldList, parseOwner, parseSourcesLine)
- dnf/rpm: edge cases for both dnf4 and dnf5 parsers, normalize/parseArch
- pacman/aur: parseUpgrades, parseGroupPkgSet, capabilities
- apk: parseUpgradeSimulation, parseListLine, SupportsDryRun
- flatpak/snap: semverCmp, stripNonNumeric edge cases
- pkg/ports: all parse functions with thorough edge cases

Every provider now has:
- Interface compliance checks (what it implements AND what it doesn't)
- Capabilities test via snack.GetCapabilities()
- Parse function unit tests with table-driven edge cases

pacman/pacman_test.go

@@ -130,6 +130,58 @@ func TestBuildArgs(t *testing.T) {
 
 func TestInterfaceCompliance(t *testing.T) {
 	var _ snack.Manager = (*Pacman)(nil)
+	var _ snack.VersionQuerier = (*Pacman)(nil)
+	var _ snack.Cleaner = (*Pacman)(nil)
+	var _ snack.FileOwner = (*Pacman)(nil)
+	var _ snack.Grouper = (*Pacman)(nil)
+	var _ snack.DryRunner = (*Pacman)(nil)
+	var _ snack.PackageUpgrader = (*Pacman)(nil)
+}
+
+func TestInterfaceNonCompliance(t *testing.T) {
+	p := New()
+	var m snack.Manager = p
+
+	if _, ok := m.(snack.Holder); ok {
+		t.Error("Pacman should not implement Holder")
+	}
+	if _, ok := m.(snack.RepoManager); ok {
+		t.Error("Pacman should not implement RepoManager")
+	}
+	if _, ok := m.(snack.KeyManager); ok {
+		t.Error("Pacman should not implement KeyManager")
+	}
+	if _, ok := m.(snack.NameNormalizer); ok {
+		t.Error("Pacman should not implement NameNormalizer")
+	}
+}
+
+func TestCapabilities(t *testing.T) {
+	caps := snack.GetCapabilities(New())
+
+	tests := []struct {
+		name string
+		got  bool
+		want bool
+	}{
+		{"VersionQuery", caps.VersionQuery, true},
+		{"Clean", caps.Clean, true},
+		{"FileOwnership", caps.FileOwnership, true},
+		{"Groups", caps.Groups, true},
+		{"DryRun", caps.DryRun, true},
+		{"Hold", caps.Hold, false},
+		{"RepoManagement", caps.RepoManagement, false},
+		{"KeyManagement", caps.KeyManagement, false},
+		{"NameNormalize", caps.NameNormalize, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.got != tt.want {
+				t.Errorf("%s = %v, want %v", tt.name, tt.got, tt.want)
+			}
+		})
+	}
 }
 
 func TestName(t *testing.T) {