Store each hash as its raw 32 bytes in a BLOB column (closes #134)

The head, tail and content columns of the files table are BLOB NOT
NULL and hold the raw 32-byte SHA-256, half the size of the hex text,
in the table and in the files_signature index. The hash phase, the
records, report and trees carry each hash as a [sha256.Size]byte and
compare it with ==; nothing is hex-encoded. Reading a record whose
hash column is not 32 bytes is an error naming its path.

In the tests, testHash is a stand-in hash: a string's bytes padded
with zeros, so fixtures keep the sort order their strings give.

Judgement call: a hash of the wrong length is refused on reading,
not copied silently.

Model: opus-5-5

report.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"crypto/sha256"
 	"database/sql"
 	"fmt"
 	"io"
@@ -24,9 +25,9 @@ const minGroupSize = 2
 type scanRec struct {
 	size    int64
 	mtime   time.Time
-	head    string
-	tail    string
-	content string
+	head    [sha256.Size]byte
+	tail    [sha256.Size]byte
+	content [sha256.Size]byte
 	path    string
 }