Feature: Multi-device screen wall on the Go server

Bring the Go web backend up to parity with the MFC server's screen wall.
A browser tab can now stream several devices over one WebSocket: the
single `watching` field becomes a per-connection watch set guarded by the
hub mutex, capped at 32 devices per socket. The challenge advertises
`caps.multiWatch` and `caps.keyframeRequest`, binary frames stay keyed by
the low 32 bits of the device ID, and connect/disconnect replies echo the
device id and request id so the wall can route concurrent responses.

Input commands (mouse, key, clipboard, rdp_reset) resolve their target
from the watch set: an explicit id must be a current subscription, and an
absent id is accepted only while exactly one device is watched. Legacy
frontends that do not send `multiWatch` keep replace-all semantics, so a
repeated device switch cannot grow the set without bound. Offline
devices are removed from every watch set, and teardown is computed from a
snapshot taken outside the hub lock to avoid re-entrant locking.

Add the `keyframe` command: it replays the cached IDR, then asks the
device to emit a fresh one, rate-limited to one request per device per
second. This exposed a Go-only corruption bug: the client answers the
request with TOKEN_KEYFRAME (134), which the dispatcher treated as a
no-op. That token only appears on forced IDRs because the client's GOP is
effectively infinite, so the dropped IDR left the browser decoding
P-frames against a reference chain it never received, producing
persistent garbling. Relay it as a keyframe instead. Also drop the
cached IDR when the resolution changes, mirroring the C++ server, so a
late joiner cannot receive an old-geometry keyframe.

Add tests for the watch-set transitions and for relaying TOKEN_KEYFRAME.

Co-Authored-By: deepseek-flash

server/go/cmd/screen_keyframe_test.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/yuanyuanxiang/SimpleRemoter/server/go/connection"
+	"github.com/yuanyuanxiang/SimpleRemoter/server/go/hub"
+	"github.com/yuanyuanxiang/SimpleRemoter/server/go/protocol"
+)
+
+type frameRecorder struct {
+	frames []recordedFrame
+}
+
+type recordedFrame struct {
+	deviceID string
+	packet   []byte
+	isKey    bool
+}
+
+func (r *frameRecorder) OnScreenFrame(deviceID string, packet []byte, isKeyframe bool) {
+	r.frames = append(r.frames, recordedFrame{deviceID, packet, isKeyframe})
+}
+func (r *frameRecorder) OnDeviceOnline(hub.DeviceInfo)       {}
+func (r *frameRecorder) OnDeviceOffline(string)              {}
+func (r *frameRecorder) OnDeviceUpdate(string, int, string)  {}
+func (r *frameRecorder) OnResolutionChange(string, int, int) {}
+func (r *frameRecorder) OnCursorChange(string, byte)         {}
+func (r *frameRecorder) OnClipboardText(string, string)      {}
+func (r *frameRecorder) OnTerminalReady(string, bool)        {}
+func (r *frameRecorder) OnTerminalData(string, []byte)       {}
+func (r *frameRecorder) OnTerminalClosed(string, string)     {}
+
+// TestOnReceiveRelaysKeyframeToken guards the forced-IDR recovery path: when
+// the browser asks for a keyframe, the client replies with TOKEN_KEYFRAME.
+// Dropping that token (the old no-op) leaves the browser decoding P-frames
+// against a reference chain it never received, i.e. persistent corruption.
+func TestOnReceiveRelaysKeyframeToken(t *testing.T) {
+	h := hub.New()
+	rec := &frameRecorder{}
+	h.Subscribe(rec)
+
+	h.Register(&hub.Device{ID: "dev", PeerIP: "10.0.0.1"},
+		&connection.Context{RemoteAddr: "10.0.0.1:1000"})
+	screenCtx := &connection.Context{RemoteAddr: "10.0.0.1:1001"}
+	if !h.BindScreenConn("dev", screenCtx) {
+		t.Fatal("screen bind failed")
+	}
+
+	// [TOKEN_KEYFRAME][algo=H264][cursorPos:8][cursorIdx:1][IDR NAL]
+	data := []byte{
+		protocol.TokenKeyframe, 2,
+		0, 0, 0, 0, 0, 0, 0, 0,
+		0,
+		0x00, 0x00, 0x00, 0x01, 0x65, 0x88, 0x84,
+	}
+	(&MyHandler{hub: h}).OnReceive(screenCtx, data)
+
+	if len(rec.frames) != 1 {
+		t.Fatalf("frames relayed = %d, want 1 (TOKEN_KEYFRAME must not be dropped)", len(rec.frames))
+	}
+	f := rec.frames[0]
+	if f.deviceID != "dev" {
+		t.Fatalf("deviceID = %q, want dev", f.deviceID)
+	}
+	if !f.isKey || f.packet[4] != 1 {
+		t.Fatalf("isKeyframe=%v frameType=%d, want true/1", f.isKey, f.packet[4])
+	}
+	if want := 9 + len(data) - (1 + protocol.ScreenFrameHeaderLen); len(f.packet) != want {
+		t.Fatalf("packet len = %d, want %d", len(f.packet), want)
+	}
+}