tsb: use the named register surface

Direct register access now reads through the named surface
(hw::mcusr::wdrf.test(), hw::ucsr0b::write(...)) instead of the string form,
matching how libavr itself is written. Zero-overhead: pure 740 B, tricks 658 B,
asm 508 B unchanged, all byte-identical across modes, protocol green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tsb/tsb_pure.cpp

@@ -201,7 +201,7 @@ void write_config()
 {
 	// A bootloader may be entered by a watchdog reset; the reference loader
 	// hands straight back to the application in that case rather than run.
-	if (avr::hw::field<"MCUSR", "WDRF">::test())
+	if (avr::hw::mcusr::wdrf.test())
 		appjump();
 
 	avr::init<serial_t>();