feat: add PackageUpgrader interface for targeted package upgrades

Add optional PackageUpgrader interface with UpgradePackages method
that upgrades specific installed packages (unlike Upgrade which
upgrades everything).

Each backend uses native upgrade commands:
- apt: apt-get install --only-upgrade
- dnf: dnf upgrade <pkg>
- pacman: pacman -S <pkg>
- apk: apk upgrade <pkg>
- pkg (FreeBSD): pkg upgrade <pkg>
- flatpak: flatpak update <pkg>
- snap: snap refresh <pkg>

Non-installed packages are filtered out and returned as Unchanged.

Closes #31

apt/apt.go

@@ -206,13 +206,21 @@ func (a *Apt) SupportsDryRun() bool { return true }
 
 // 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.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)
 )
+
+// UpgradePackages upgrades specific installed packages.
+func (a *Apt) UpgradePackages(ctx context.Context, pkgs []snack.Target, opts ...snack.Option) (snack.InstallResult, error) {
+	a.Lock()
+	defer a.Unlock()
+	return upgradePackages(ctx, pkgs, opts...)
+}

apt/apt_linux.go

@@ -211,3 +211,50 @@ func version(ctx context.Context, pkg string) (string, error) {
 	}
 	return v, nil
 }
+
+func upgradePackages(ctx context.Context, pkgs []snack.Target, opts ...snack.Option) (snack.InstallResult, error) {
+	o := snack.ApplyOptions(opts...)
+	var toUpgrade []snack.Target
+	var unchanged []string
+	for _, t := range pkgs {
+		if o.DryRun {
+			toUpgrade = append(toUpgrade, t)
+			continue
+		}
+		ok, err := isInstalled(ctx, t.Name)
+		if err != nil {
+			return snack.InstallResult{}, err
+		}
+		if !ok {
+			unchanged = append(unchanged, t.Name)
+		} else {
+			toUpgrade = append(toUpgrade, t)
+		}
+	}
+	if len(toUpgrade) > 0 {
+		// Use --only-upgrade to ensure we don't install new packages.
+		args := buildArgs("install", toUpgrade, opts...)
+		idx := -1
+		for i, a := range args {
+			if a == "install" {
+				idx = i
+				break
+			}
+		}
+		if idx >= 0 {
+			args = append(args[:idx+1], append([]string{"--only-upgrade"}, args[idx+1:]...)...)
+		}
+		cmd := exec.CommandContext(ctx, args[0], args[1:]...)
+		var stderr bytes.Buffer
+		cmd.Stderr = &stderr
+		if err := cmd.Run(); err != nil {
+			return snack.InstallResult{}, fmt.Errorf("apt-get install --only-upgrade: %w: %s", err, stderr.String())
+		}
+	}
+	var upgraded []snack.Package
+	for _, t := range toUpgrade {
+		v, _ := version(ctx, t.Name)
+		upgraded = append(upgraded, snack.Package{Name: t.Name, Version: v, Installed: true})
+	}
+	return snack.InstallResult{Installed: upgraded, Unchanged: unchanged}, nil
+}

apt/apt_other.go

@@ -49,3 +49,7 @@ func isInstalled(_ context.Context, _ string) (bool, error) {
 func version(_ context.Context, _ string) (string, error) {
 	return "", snack.ErrUnsupportedPlatform
 }
+
+func upgradePackages(_ context.Context, _ []snack.Target, _ ...snack.Option) (snack.InstallResult, error) {
+	return snack.InstallResult{}, snack.ErrUnsupportedPlatform
+}