Implement plugin catalog and discovery system

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

internal/plugin/update.go

@@ -0,0 +1,85 @@
+/*
+Copyright 2026 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package plugin
+
+const (
+	SkipReasonManual   = "manually installed"
+	SkipReasonUpToDate = "already up to date"
+)
+
+// UpdateResult represents the outcome of updating a single plugin.
+// When an update is available, Manifest, Version and Platform are
+// populated so the caller can install without re-fetching or re-resolving.
+type UpdateResult struct {
+	Name        string
+	FromVersion string
+	ToVersion   string
+	Skipped     bool
+	SkipReason  string
+	Manifest    *PluginManifest
+	Version     *PluginVersion
+	Platform    *PluginPlatform
+	Err         error
+}
+
+// CheckUpdate compares the installed version against the latest in the catalog.
+// Returns an UpdateResult describing what should happen. When an update is
+// available, Manifest is populated so the caller can install without re-fetching.
+func CheckUpdate(pluginDir string, name string, catalog *CatalogClient, goos, goarch string) UpdateResult {
+	receipt := ReadReceipt(pluginDir, name)
+	if receipt == nil {
+		return UpdateResult{
+			Name:       name,
+			Skipped:    true,
+			SkipReason: SkipReasonManual,
+		}
+	}
+
+	manifest, err := catalog.FetchManifest(name)
+	if err != nil {
+		return UpdateResult{Name: name, Err: err}
+	}
+
+	latest, err := ResolveVersion(manifest, "")
+	if err != nil {
+		return UpdateResult{Name: name, Err: err}
+	}
+
+	if receipt.Version == latest.Version {
+		return UpdateResult{
+			Name:        name,
+			FromVersion: receipt.Version,
+			ToVersion:   latest.Version,
+			Skipped:     true,
+			SkipReason:  SkipReasonUpToDate,
+		}
+	}
+
+	plat, err := ResolvePlatform(latest, goos, goarch)
+	if err != nil {
+		return UpdateResult{Name: name, Err: err}
+	}
+
+	return UpdateResult{
+		Name:        name,
+		FromVersion: receipt.Version,
+		ToVersion:   latest.Version,
+		Manifest:    manifest,
+		Version:     latest,
+		Platform:    plat,
+	}
+}