Hash every file's head, tail and content in the hash phase (closes #132)

Every file the walk records now gets three hashes in the hash phase:
head and tail, of its first and last 64 KiB, and content. The content
hash covers the whole file under 100 MiB, and 1 MiB at each gigabyte
from 100 MiB up, so a large file is never read in full. One 100 MiB
constant replaces the 10 MiB and 50 MiB thresholds. The size census,
the records stored without hashes and the content phase are gone, so a
scan has four phases and a rescan reads only new and changed files.
report and trees no longer special-case a missing content hash. README
"Duplicate detection" and "Rationale" are rewritten.

Judgement call: info drops its two hash-count lines, which would now
always equal the record count.

Model: opus-5-5

trees.go

@@ -286,18 +286,8 @@ func (b *treeBuilder) finish() (*treeNode, []*treeNode) {
 // name and its signature (size, head, tail, content); mtime is
 // excluded.
 func fileEntry(name string, r scanRec) string {
-	content := r.content
-
-	// 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 content == "" {
-		content = "unhashed\x00" + r.path
-	}
-
 	return "f\x00" + name + "\x00" + strconv.FormatInt(r.size, 10) +
-		"\x00" + r.head + "\x00" + r.tail + "\x00" + content
+		"\x00" + r.head + "\x00" + r.tail + "\x00" + r.content
 }
 
 // collectTreeGroups groups directories by digest, counting each one on