Store mtime in nanoseconds so a same-second rewrite is re-hashed (closes #12)

scan recorded mtime in whole seconds, so a file rewritten in place at
the same size within the same second as its recorded mtime was classed
unchanged and kept its old hashes. The walk, a file given as an operand,
and the content phase's recheck now all use ModTime().UnixNano(); the
change rule itself is unchanged. The column stays INTEGER and
PRAGMA user_version stays 1, per the owner's ruling. A database written
by an earlier build holds seconds, so its files count as changed once,
at the next scan that covers them. README states the new resolution.

Model: opus-5-5

scan.go

@@ -807,7 +807,7 @@ func unchangedFile(r scanRec) (fileRec, bool, error) {
 	}
 
 	if !fi.Mode().IsRegular() || fi.Size() != r.size ||
-		fi.ModTime().Unix() > r.mtime {
+		fi.ModTime().UnixNano() > r.mtime {
 		return fileRec{}, false, nil
 	}
 
@@ -966,7 +966,7 @@ func seedRoot(ctx context.Context, root string,
 	sendEvent(ctx, events, walkEvent{rec: fileRec{
 		path:  root,
 		size:  fi.Size(),
-		mtime: fi.ModTime().Unix(),
+		mtime: fi.ModTime().UnixNano(),
 		dev:   dev,
 		ino:   ino,
 	}})
@@ -1124,7 +1124,7 @@ func emitFile(ctx context.Context, p string, e fs.DirEntry,
 	sendEvent(ctx, events, walkEvent{rec: fileRec{
 		path:  p,
 		size:  info.Size(),
-		mtime: info.ModTime().Unix(),
+		mtime: info.ModTime().UnixNano(),
 		dev:   dev,
 		ino:   ino,
 	}})