Implement first AVR emulator backend with SimAVR and track avr8js follow-on #24

Goal

Implement the first AVR emulator backend so fbuild can run post-build smoke tests without hardware.

Why

AVR is the best first target:

• many AVR board definitions already advertise simavr
• AVR artifacts naturally produce firmware.elf
• UART/text-driven smoke tests map well to current monitor semantics

avr8js should also be tracked as a relevant implementation family for future Node/browser-backed smoke testing, even if simavr is the first native backend delivered.

Scope

• implement SimavrRunner
• support Uno first, then Mega and Leonardo
• consume firmware.elf from normal build output
• integrate stdout/serial output with pass/fail monitor classification
• document host support limits, especially Windows constraints
• evaluate where avr8js fits after the native runner path exists

Acceptance criteria

• fbuild test-emu works end-to-end for at least Uno via simavr
• pass/fail/timeout behavior is covered by tests or fixtures
• unsupported host environments fail explicitly
• follow-up notes capture whether avr8js should be a separate runner or tooling path

References

• wokwi/avr8js: https://github.com/wokwi/avr8js
• PlatformIO SimAVR docs: https://docs.platformio.org/en/stable/advanced/unit-testing/simulators/simavr.html

Parent: #21