v0.2.1

Fix: serial capture after flash now waits for the flash_done event the coordinator sends on the serial WebSocket, instead of polling session state. Previously flash() returned immediately (the eager serial connection had already marked the session active), so read()/expect() ran while the board was still mid-flash and timed out empty. flash() now blocks until flashing truly completes and flushes pre-flash output, so the freshly-flashed firmware's serial is reliably captured. Verified on esp32-s3, stm32-h753, stm32-f446, rp2350.