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

trees.go

@@ -127,10 +127,8 @@ func buildHierarchy(recs []scanRec) (*treeNode, []*treeNode) {
 			size: r.size, head: r.head, tail: r.tail, content: r.content,
 		}
 
-		// A record without a content hash (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) has unknown
-		// content: give it a signature no other file can share, so
+		// A record without a content hash has unknown content (README
+		// "Database"): give it a signature no other file can share, so
 		// trees containing it never compare equal. Real hashes are
 		// hex, so the NUL-prefixed form cannot collide.
 		if sig.content == "" {