feat: add flatpak and snap package manager implementations

flatpak: implements Manager, Cleaner, and RepoManager interfaces
- Install/Remove/Purge/Upgrade with flatpak CLI
- Repository management (add/remove/list remotes)
- Autoremove unused runtimes

snap: implements Manager and VersionQuerier interfaces
- Install with --classic/--channel support via Target.FromRepo
- Remove/Purge/Upgrade via snap CLI
- Version queries with semver comparison
- Upgrade availability via snap refresh --list

Both packages follow the existing pattern:
- Exported methods on struct delegate to unexported functions
- _linux.go for real implementation, _other.go stubs
- Compile-time interface checks
- Parser tests for all output formats

flatpak/flatpak_other.go

@@ -0,0 +1,63 @@
+//go:build !linux
+
+package flatpak
+
+import (
+	"context"
+
+	"github.com/gogrlx/snack"
+)
+
+func available() bool { return false }
+
+func install(_ context.Context, _ []snack.Target, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func remove(_ context.Context, _ []snack.Target, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func purge(_ context.Context, _ []snack.Target, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func upgrade(_ context.Context, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func list(_ context.Context) ([]snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func search(_ context.Context, _ string) ([]snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func info(_ context.Context, _ string) (*snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func isInstalled(_ context.Context, _ string) (bool, error) {
+	return false, snack.ErrUnsupportedPlatform
+}
+
+func version(_ context.Context, _ string) (string, error) {
+	return "", snack.ErrUnsupportedPlatform
+}
+
+func autoremove(_ context.Context, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func listRepos(_ context.Context) ([]snack.Repository, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func addRepo(_ context.Context, _ snack.Repository) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func removeRepo(_ context.Context, _ string) error {
+	return snack.ErrUnsupportedPlatform
+}