feat: Denkovi USB 4/8 (selectable) + generic USB-serial relay board (CH340/LCUS)

- Denkovi (FT245 D2XX) now selectable as 4 or 8 relays (a 4-relay board just uses
  the low nibble); NewDenkovi takes a count, driven by the device's Channels.
- New 'usbrelay' backend: cheap CH340/LCUS USB-serial boards over a COM port
  using the common A0 protocol ([0xA0][relay][state][checksum]); write-only, so
  Status is a shadow. Channel count configurable (1/2/4/8/16).
- StationDevice gains Channels; deviceRelayCount() drives label/relay counts for
  the variable-size types. Device editor: channel selector + COM-port picker
  (usbrelay) / FTDI-serial (denkovi).

Both untested on hardware; the A0 command set / Denkovi bit order may need a tweak
after a live test.

internal/relaydev/denkovi_other.go

@@ -10,12 +10,17 @@ import (
 	"fmt"
 )
 
-type denkoviStub struct{}
+type denkoviStub struct{ count int }
 
 // NewDenkovi returns a stub on non-Windows builds.
-func NewDenkovi(serial string) Device { return denkoviStub{} }
+func NewDenkovi(serial string, count int) Device {
+	if count != 4 {
+		count = 8
+	}
+	return denkoviStub{count: count}
+}
 
-func (denkoviStub) Count() int { return 8 }
+func (s denkoviStub) Count() int { return s.count }
 func (denkoviStub) Status(context.Context) ([]bool, error) {
 	return nil, fmt.Errorf("Denkovi USB relay board is only supported on Windows")
 }

internal/relaydev/denkovi_windows.go

@@ -46,10 +46,14 @@ type denkovi struct {
 	opened bool
 }
 
-// NewDenkovi builds a driver for a Denkovi USB 8-relay board identified by its
-// FTDI serial number (shown by the vendor tool / FT_PROG, e.g. "DAE0006K").
-func NewDenkovi(serial string) Device {
-	return &denkovi{serial: strings.TrimSpace(serial), count: 8}
+// NewDenkovi builds a driver for a Denkovi USB relay board (4 or 8 relays)
+// identified by its FTDI serial number (shown by the vendor tool / FT_PROG,
+// e.g. "DAE0006K"). count defaults to 8; a 4-relay board just uses the low 4 bits.
+func NewDenkovi(serial string, count int) Device {
+	if count != 4 && count != 8 {
+		count = 8
+	}
+	return &denkovi{serial: strings.TrimSpace(serial), count: count}
 }
 
 func (d *denkovi) Count() int { return d.count }

internal/relaydev/serialrelay.go

@@ -0,0 +1,89 @@
+package relaydev
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"sync"
+
+	"go.bug.st/serial"
+)
+
+// serialRelay drives the common cheap USB-serial relay boards (CH340 / LCUS-1
+// style) that use the "A0" command protocol over a virtual COM port:
+//
+//	[0xA0] [relay 1-based] [state 0|1] [checksum]     checksum = (0xA0+relay+state) & 0xFF
+//
+// e.g. relay 1 ON = A0 01 01 A2, relay 1 OFF = A0 01 00 A1. These boards are
+// write-only (no state readback), so Status reflects a shadow of what we sent.
+// Channel count varies by board (1/2/4/8/16), so it is configurable.
+type serialRelay struct {
+	portName string
+	count    int
+	baud     int
+	mu       sync.Mutex
+	port     serial.Port
+	shadow   []bool
+}
+
+// NewSerialRelay builds a driver for a CH340/LCUS-style USB-serial relay board on
+// the given COM port with `count` channels (defaults to 8).
+func NewSerialRelay(port string, count int) Device {
+	if count < 1 {
+		count = 8
+	}
+	return &serialRelay{portName: strings.TrimSpace(port), count: count, baud: 9600, shadow: make([]bool, count)}
+}
+
+func (s *serialRelay) Count() int { return s.count }
+
+func (s *serialRelay) ensureOpen() error {
+	if s.port != nil {
+		return nil
+	}
+	if s.portName == "" {
+		return fmt.Errorf("no COM port set for the USB relay board")
+	}
+	p, err := serial.Open(s.portName, &serial.Mode{BaudRate: s.baud})
+	if err != nil {
+		return fmt.Errorf("open %s: %w", s.portName, err)
+	}
+	s.port = p
+	return nil
+}
+
+func (s *serialRelay) Set(ctx context.Context, relay int, on bool) error {
+	if relay < 1 || relay > s.count {
+		return fmt.Errorf("relay %d out of range 1..%d", relay, s.count)
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if err := s.ensureOpen(); err != nil {
+		return err
+	}
+	st := byte(0)
+	if on {
+		st = 1
+	}
+	r := byte(relay)
+	frame := []byte{0xA0, r, st, byte(0xA0) + r + st} // last byte = checksum
+	if _, err := s.port.Write(frame); err != nil {
+		// Drop the handle so the next call reopens (USB unplugged / port reset).
+		s.port.Close()
+		s.port = nil
+		return fmt.Errorf("write to %s: %w", s.portName, err)
+	}
+	s.shadow[relay-1] = on
+	return nil
+}
+
+func (s *serialRelay) Status(ctx context.Context) ([]bool, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if err := s.ensureOpen(); err != nil {
+		return nil, err
+	}
+	out := make([]bool, s.count)
+	copy(out, s.shadow)
+	return out, nil
+}