Add deterministic UART decoder unit tests for device/serial

[Copilot]

This PR adds a focused test suite for device/serial to cover UART frame decoding behavior without touching production code or requiring a live PTY. It validates normal decode paths and key error/re-sync paths across relevant config combinations.

• Scope

  • Adds device/serial/uart_test.go only.
  • No changes to uart.go or pty.go.

• Frame sample builder for decode-driven tests

  • Introduces a helper that emits oversampled serial line levels from (Config, value).
  • Encodes start/data/parity/stop bits with support for:
    • DataBits (5/7/8)
    • StopBits (1/2)
    • Parity (None/Even/Odd)
    • LsbLast and IdleLow
  • Parity generation matches decoder convention (parityAcc seeded from ParityOdd).

• Round-trip decode matrix coverage

  • Table-driven decode tests feed generated samples into WriteSample and assert:
    • frame detection (gotFrame)
    • decoded value correctness under all combinations above

• Negative-path coverage

  • Parity mismatch frame → asserts "parity error".
  • Bad stop bit (space instead of mark) → asserts "framing error".
  • Start-bit glitch (line returns high before center sample) → asserts no frame produced.

• Utility coverage

  • Adds a direct unit test for bool2int(true)==1 and bool2int(false)==0.

samples := emitFrameSamples(*cfg, value, nil, nil)
got, gotFrame, err := feedSamples(u, samples)
if err != nil || !gotFrame || got != want {
	t.Fatalf("cfg=%+v value=0x%x got=%#x frame=%v err=%v want=%#x",
		*cfg, value, got, gotFrame, err, want)
}