Compute the content hash only when head and tail match (closes #61)

A file of 10 MiB or more now gets only its head and tail in the hash
phase. A new content phase, after the update phase, finds every record
of that size without a content hash whose size, head and tail match
another record's, anywhere in the database, checks each file with
lstat, and reads a group only while at least two members remain. It
reuses the hash worker pool, now given its hash function. report and
trees leave out records without a content hash. The README, help text
and TODO entry describe the gate; the schema stays at version 1.

Model: opus-5-5

report.go

@@ -117,10 +117,11 @@ func collectDupeGroups(recs []scanRec) []dupeGroup {
 	groups := make(map[fileSig][]string)
 
 	for _, r := range recs {
-		// A record without hashes (its size was unique when last
-		// scanned) has unknown content and is never reported as a
-		// duplicate.
-		if r.head == "" {
+		// 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.
+		if r.content == "" {
 			continue
 		}