feat(stdlib): add String_Chars, String_FromBool, fix Fmt and Net

- String.bux: add String_Chars(s, index) for char-at-index access
- String.bux: add String_FromBool(b) conversion
- Fmt.bux: rewrite with correct stdlib imports (was using non-existent names)
- Net.bux: fix String_Len -> bux_strlen (String_Len not imported)
- All lib modules compile successfully

lib/Net.bux

@@ -42,7 +42,7 @@ func Net_Connect(fd: int, addr: String, port: int) -> bool {
 
 /* Send data. Returns bytes sent or -1 on error. */
 func Net_Send(fd: int, data: String) -> int {
-    return bux_socket_send(fd, data, String_Len(data) as int);
+    return bux_socket_send(fd, data, bux_strlen(data) as int);
 }
 
 /* Receive up to maxLen bytes. Returns empty string on error/EOF. */