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

cancel_test.go

@@ -456,7 +456,7 @@ func TestHashWorkerDropsQueuedRuns(t *testing.T) {
 	go func() {
 		defer close(done)
 
-		hashWorker(cancelledContext(t), jobs, results)
+		hashWorker(cancelledContext(t), jobs, results, hashSignature)
 	}()
 
 	awaitReturn(t, done, "hashWorker")