fix(uno): drain stray UART RX bytes after Serial.begin()

[dim20]

Problem

After boot (or any rurp_set_communication_mode() call), the Uno's COBS decoder in loop() sometimes picks up a stray byte that doesn't form a valid frame. This triggers MSG_ERR_EMPTY_INPUT, which on the host side arrives before the real OK ack and causes the connection probe to fail.

The host sees:

Fix

Add a 100 ms settle delay after the initial UART RX flush in rurp_set_communication_mode(), then a second flush. This catches stray bytes that arrive after the first flush regardless of their origin.

Root cause note

The exact source of the stray byte has not been confirmed with a logic analyser. It is likely a late byte from the ATmega16U2 USB-serial bridge that lands after the initial flush, but could also be an electrical glitch on PD0 during the DDR transition. The settle delay + second flush is harmless either way.

Tested

Bench-tested on Uno R3 (ATmega16U2) with firmware 3.0.0b10 — the stale MSG_ERR_EMPTY_INPUT messages no longer appear.

[gemini-code-assist]

Code Review

This pull request adds a 100ms delay and a second serial port flush in src/boards/uno_rurp_shield.cpp to handle stray bytes from the USB-serial bridge during UART initialization. Feedback suggests that this double-flush pattern is redundant and introduces a race condition where legitimate host data could be discarded during the delay; it is recommended to use a single delay followed by a single flush instead.

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