fix(detector): sort previous JSON dirs in descending order for diff (#2555)

* fix(detector): sort previous JSON dirs in descending order for diff

ListValidJSONDirs in detector was switched from a descending sort.Slice
to slices.Sort in #2415, which sorts ascending. The doc comment and
loadPrevious both assume descending order — loadPrevious takes dirs[1:]
expecting the second-newest dir as the previous scan, but with the
ascending sort it picks the second-oldest, so report --diff-plus
compares against an ancient scan instead of the latest prior one.

Match reporter/util.go's ListValidJSONDirs by sorting descending.

Reported in #2536

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* test(detector): regression test for ListValidJSONDirs sort order

Reproduces the scenario from discussion #2536 — when many timestamped
result dirs exist, the newest must come first so loadPrevious picks
the second-newest as the previous scan.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MaineK00n

detector/util.go

@@ -3,6 +3,7 @@
 package detector
 
 import (
+	"cmp"
 	"encoding/json"
 	"fmt"
 	"maps"
@@ -235,7 +236,9 @@ func ListValidJSONDirs(resultsDir string) (dirs []string, err error) {
 			}
 		}
 	}
-	slices.Sort(dirs)
+	slices.SortFunc(dirs, func(a, b string) int {
+		return -cmp.Compare(a, b)
+	})
 	return
 }
 

detector/util_test.go

@@ -0,0 +1,63 @@
+//go:build !scanner
+
+package detector
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestListValidJSONDirs_SortedDescending(t *testing.T) {
+	root := t.TempDir()
+
+	names := []string{
+		"2026-04-24T08-22-31+0000",
+		"2026-04-25T08-27-25+0000",
+		"2026-04-25T17-52-14+0000",
+		"2026-04-25T18-20-55+0000",
+		"2026-04-25T19-48-34+0000",
+		"2026-04-27T13-02-05+0000",
+		"2026-04-28T12-56-45+0000",
+		"2026-04-29T12-57-26+0000",
+		"2026-04-30T12-57-38+0000",
+		"2026-05-01T12-57-29+0000",
+		"2026-05-01T15-17-13+0000",
+		"2026-05-01T15-33-21+0000",
+		"not-a-timestamp",
+	}
+	for _, n := range names {
+		if err := os.Mkdir(filepath.Join(root, n), 0o755); err != nil {
+			t.Fatalf("mkdir %s: %v", n, err)
+		}
+	}
+
+	got, err := ListValidJSONDirs(root)
+	if err != nil {
+		t.Fatalf("ListValidJSONDirs: %v", err)
+	}
+
+	want := []string{
+		filepath.Join(root, "2026-05-01T15-33-21+0000"),
+		filepath.Join(root, "2026-05-01T15-17-13+0000"),
+		filepath.Join(root, "2026-05-01T12-57-29+0000"),
+		filepath.Join(root, "2026-04-30T12-57-38+0000"),
+		filepath.Join(root, "2026-04-29T12-57-26+0000"),
+		filepath.Join(root, "2026-04-28T12-56-45+0000"),
+		filepath.Join(root, "2026-04-27T13-02-05+0000"),
+		filepath.Join(root, "2026-04-25T19-48-34+0000"),
+		filepath.Join(root, "2026-04-25T18-20-55+0000"),
+		filepath.Join(root, "2026-04-25T17-52-14+0000"),
+		filepath.Join(root, "2026-04-25T08-27-25+0000"),
+		filepath.Join(root, "2026-04-24T08-22-31+0000"),
+	}
+
+	if len(got) != len(want) {
+		t.Fatalf("len(got)=%d want=%d\ngot=%v", len(got), len(want), got)
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Errorf("dirs[%d] = %s, want %s", i, got[i], want[i])
+		}
+	}
+}