Check every group member and show the content phase from its start

The content phase now checks every record sharing a size, head and
tail with lstat, including those that already have a content hash, and
reads those without one only when at least two pass. Only a missing
file is passed over silently; any other lstat error is warned about and
counted as skipped. The phase shows a running count while it queries
and checks, then its bar. The README states once when content is empty.
New tests cover these and hard links.

Model: opus-5-5

report.go

@@ -118,9 +118,7 @@ func collectDupeGroups(recs []scanRec) []dupeGroup {
 
 	for _, r := range recs {
 		// A record without a content hash has unknown content and is
-		// never reported as a duplicate: its size was unique when last
-		// scanned, or it is headTailMin or more and has not yet matched
-		// another record on size, head, and tail.
+		// never reported as a duplicate (README "Database").
 		if r.content == "" {
 			continue
 		}