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

apt/apt_test.go

@@ -67,5 +67,49 @@ func TestNew(t *testing.T) {
 	}
 }
 
-// Verify Apt implements snack.Manager at compile time.
-var _ snack.Manager = (*Apt)(nil)
+func TestSupportsDryRun(t *testing.T) {
+	a := New()
+	if !a.SupportsDryRun() {
+		t.Error("expected SupportsDryRun() = true")
+	}
+}
+
+func TestCapabilities(t *testing.T) {
+	caps := snack.GetCapabilities(New())
+	checks := []struct {
+		name string
+		got  bool
+		want bool
+	}{
+		{"VersionQuery", caps.VersionQuery, true},
+		{"Hold", caps.Hold, true},
+		{"Clean", caps.Clean, true},
+		{"FileOwnership", caps.FileOwnership, true},
+		{"RepoManagement", caps.RepoManagement, true},
+		{"KeyManagement", caps.KeyManagement, true},
+		{"Groups", caps.Groups, false},
+		{"NameNormalize", caps.NameNormalize, true},
+		{"DryRun", caps.DryRun, true},
+	}
+	for _, c := range checks {
+		t.Run(c.name, func(t *testing.T) {
+			if c.got != c.want {
+				t.Errorf("Capabilities.%s = %v, want %v", c.name, c.got, c.want)
+			}
+		})
+	}
+}
+
+// Compile-time interface checks.
+var (
+	_ snack.Manager         = (*Apt)(nil)
+	_ snack.VersionQuerier  = (*Apt)(nil)
+	_ snack.Holder          = (*Apt)(nil)
+	_ snack.Cleaner         = (*Apt)(nil)
+	_ snack.FileOwner       = (*Apt)(nil)
+	_ snack.RepoManager     = (*Apt)(nil)
+	_ snack.KeyManager      = (*Apt)(nil)
+	_ snack.NameNormalizer  = (*Apt)(nil)
+	_ snack.DryRunner       = (*Apt)(nil)
+	_ snack.PackageUpgrader = (*Apt)(nil)
+)