feat(tci): receive audio over the TCI WebSocket (experimental)

A SunSDR already carries its receive audio on the same WebSocket as its
commands, so a virtual audio cable and a second sound card are two pieces
of plumbing an operator installs for no reason. This is the receive half:
what the QSO recorder and the CW decoder need.

The reader now looks at the frame type. It used to ignore it and split
every frame on ';' -- harmless only for as long as no stream was ever
opened, since audio bytes would otherwise have been handed to the command
parser a hundred times a second.

NOTHING HERE IS CONFIRMED ON A RADIO. The header layout comes from the
TCI documentation, and the stream-type numbers are exactly the sort of
detail a document gets right and a memory of it does not -- so the first
forty frames of a session are logged verbatim, and a test bench in
Preferences > Audio reports the sample rate the radio chose, the frames
arriving and the peak level of the last second. 'The stream is open' and
'audio is arriving' are different claims and only the second is worth
anything to whoever tries this first.

Transmit (the voice keyer) is the other half and is deliberately absent:
it has to answer the radio's chrono packets at the right pace, and that
is worth doing once the format is settled on real hardware.

internal/cat/tci_manager.go

@@ -0,0 +1,39 @@
+//go:build windows
+
+package cat
+
+import "fmt"
+
+// TCIAudioController is the receive-audio capability of the TCI backend, kept
+// as an interface for the same reason as the Flex and Yaesu ones: the host asks
+// the manager, and a station running something else gets a clear "this backend
+// does not do that" instead of a nil dereference.
+type TCIAudioController interface {
+	StartTCIAudio(rx, rate int) error
+	StopTCIAudio() error
+	TCIAudioStatus() TCIAudioStatus
+}
+
+// TCIAudioState returns the stream's state, or (zero, false) when the active
+// backend is not a TCI radio.
+func (m *Manager) TCIAudioState() (TCIAudioStatus, bool) {
+	m.mu.RLock()
+	b := m.backend
+	m.mu.RUnlock()
+	if tc, ok := b.(TCIAudioController); ok {
+		return tc.TCIAudioStatus(), true
+	}
+	return TCIAudioStatus{}, false
+}
+
+// TCIAudioDo dispatches an audio command onto the CAT goroutine, like every
+// other backend-specific control.
+func (m *Manager) TCIAudioDo(fn func(TCIAudioController) error) error {
+	return m.exec(func(b Backend) error {
+		tc, ok := b.(TCIAudioController)
+		if !ok {
+			return fmt.Errorf("active CAT backend is not a TCI radio")
+		}
+		return fn(tc)
+	})
+}