Make memfile offset atomic so checkdisk() can read it from other threads

checkdisk() is called from the parsing threads and sums the write
positions of every reader. The geometry/index/vertex/node positions were
already std::atomic, but memfile::off (the string pool and tree sizes)
was a plain size_t that the owning thread updates in memfile_write(),
which ThreadSanitizer reports as a data race.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EfWkLhwEvKQoNGVPsLGehj

memfile.hpp

@@ -10,7 +10,8 @@ struct memfile {
 	std::string map;
 	unsigned long tree = 0;
 	FILE *fp = NULL;
-	size_t off = 0;
+	// Read by checkdisk() from other threads while the owning reader thread appends
+	std::atomic<size_t> off{0};
 };
 
 struct memfile *memfile_open(int fd);