Adopt cc-lib hierarchical JobData/Statistics structs

cc-lib changed JobData, ScopedJobStats and Job.Statistics from flat maps
to structs (a .Metrics map plus array-valued Groups) to represent
filesystem (and future interconnect) metric groups. Migrate all
construction, indexing and iteration to the new API across the archive
backends, metricstore query path, metric dispatcher, archiver, importer,
tagger, taskmanager, repository and API layers.

Semantics: DecodeJobStats now projects JobData.Groups into
ScopedJobStats.Groups, and the archiver derives per-filesystem node-scope
statistics into Job.Statistics.Groups. deepCopy, resampling and the
metric/scope filter are group-aware. The metricstore internal storage
(buffers, selector tree, checkpoint/parquet) is unchanged; all conversion
stays at the LoadData/archive-codec seam.

Note: requires the corresponding cc-lib release; bump the cc-lib
dependency version once tagged (a local go.mod replace was used during
development and is intentionally not committed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/archive/fsBackend_test.go

@@ -129,7 +129,7 @@ func TestLoadJobData(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	for _, scopes := range data {
+	for _, scopes := range data.Metrics {
 		// fmt.Printf("Metric name: %s\n", name)
 
 		if _, exists := scopes[schema.MetricScopeNode]; !exists {