feat(aur): implement native AUR client

Native Go implementation using the AUR RPC v5 API for search/info
queries and git+makepkg+pacman for building and installing packages.

Implements:
- Manager (full: Install, Remove, Purge, Upgrade, Update, List, Search,
  Info, IsInstalled, Version)
- VersionQuerier (LatestVersion, ListUpgrades, UpgradeAvailable, VersionCmp)
- Cleaner (Autoremove via pacman, Clean build dir)
- PackageUpgrader (UpgradePackages)

Key design decisions:
- No CLI wrapper (paru/yay) — uses AUR RPC API directly + git clone
- Packages are built with makepkg and installed via pacman -U
- Foreign packages (pacman -Qm) are treated as AUR packages for List
- Batch RPC queries (rpcInfoMulti) for efficient upgrade checks
- Configurable build directory and makepkg flags
- Not added to detect.Default() candidates (AUR supplements pacman,
  not replaces it) but available via detect.ByName("aur")

aur/capabilities.go

@@ -0,0 +1,54 @@
+package aur
+
+import (
+	"context"
+
+	"github.com/gogrlx/snack"
+)
+
+// Compile-time interface checks.
+var (
+	_ snack.Manager         = (*AUR)(nil)
+	_ snack.VersionQuerier  = (*AUR)(nil)
+	_ snack.Cleaner         = (*AUR)(nil)
+	_ snack.PackageUpgrader = (*AUR)(nil)
+)
+
+// LatestVersion returns the latest version available in the AUR.
+func (a *AUR) LatestVersion(ctx context.Context, pkg string) (string, error) {
+	return latestVersion(ctx, pkg)
+}
+
+// ListUpgrades returns installed foreign packages that have newer versions in the AUR.
+func (a *AUR) ListUpgrades(ctx context.Context) ([]snack.Package, error) {
+	return listUpgrades(ctx)
+}
+
+// UpgradeAvailable reports whether a newer version is available in the AUR.
+func (a *AUR) UpgradeAvailable(ctx context.Context, pkg string) (bool, error) {
+	return upgradeAvailable(ctx, pkg)
+}
+
+// VersionCmp compares two version strings using pacman's vercmp.
+func (a *AUR) VersionCmp(ctx context.Context, ver1, ver2 string) (int, error) {
+	return versionCmp(ctx, ver1, ver2)
+}
+
+// Autoremove removes orphaned packages via pacman.
+func (a *AUR) Autoremove(ctx context.Context, opts ...snack.Option) error {
+	a.Lock()
+	defer a.Unlock()
+	return autoremove(ctx, opts...)
+}
+
+// Clean removes cached build artifacts from the build directory.
+func (a *AUR) Clean(_ context.Context) error {
+	return a.cleanBuildDir()
+}
+
+// UpgradePackages rebuilds and reinstalls specific AUR packages.
+func (a *AUR) UpgradePackages(ctx context.Context, pkgs []snack.Target, opts ...snack.Option) (snack.InstallResult, error) {
+	a.Lock()
+	defer a.Unlock()
+	return a.install(ctx, pkgs, opts...)
+}