fix(avr8js): test-emu hangs after stopping the nodejs-wheel launcher #1694

AVR8JS runs firmware correctly under fbuild 2.5.42, but test-emu never returns after a halt marker or emulator timeout when node is the Python entry point installed by nodejs-wheel.

Reproduced from FastLED with:

uv run fbuild test-emu --emulator avr8js --environment uno --timeout 30 --halt-on-success 'Test loop' --expect 'Test setup starting' .build/fbuild/uno

The daemon records both expected markers within one second. The CLI remains blocked. A second run without a matching halt marker remains blocked beyond --timeout 10.

The process tree explains the failure:

fbuild-daemon
  python .venv/bin/node headless.mjs ...
    nodejs_wheel/bin/node headless.mjs ...

run_avr8js_headless kills the launcher with Tokio Child::kill. Native Node survives, is reparented, and holds the inherited stdout/stderr pipes open. The runner waits indefinitely for its line-reader tasks. Manually terminating Node releases the CLI, which reports the already-determined success or timeout result.

Resolution in #1697: discover the absolute native Node executable using node --print process.execPath and launch it directly for headless emulation. Both production entry points use this shared discovery function, so the owned child is the emulator process. Real-Node regressions under FastLED's nodejs-wheel environment cover executable identity and success/error/timeout shutdown. Uno firmware success and timeout runs also finish automatically.

Related historical workaround: FastLED/FastLED#2645 and FastLED/FastLED#2651. Coordinated with FastLED/FastLED#4884.