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

Every file the walk records now gets three hashes: 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 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 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