feat: enhance plugin system with Go helpers and add Modbus plugin

- Register Go helper functions (modbus.crc16, hex.encode/decode,
  util.bytes) into Lua states for Modbus RTU support
- Add plugins/modbus.lua with .modbus read/write commands
- Fix Reload race condition (hold lock across Unload+Load)
- Make App.Close nil-safe for sess
- Restore internal/console/console_test.go

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/app/app.go

@@ -182,7 +182,9 @@ func (a *App) Close() {
 		close(a.done)
 		a.forward.Close()
 		a.plugins.Close()
-		a.sess.Close()
+		if a.sess != nil {
+			a.sess.Close()
+		}
 		if a.logFile != nil {
 			_ = a.logFile.Close()
 		}

internal/console/console_test.go

@@ -0,0 +1,83 @@
+package console
+
+import (
+	"testing"
+
+	"github.com/jixishi/SerialTerminalForWindowsTerminal/internal/config"
+)
+
+func TestParseCSIu(t *testing.T) {
+	tests := []struct {
+		name string
+		seq  []byte
+		cp   int
+		mod  int
+		ok   bool
+	}{
+		{name: "ctrl+alt+c lowercase", seq: []byte{0x1b, '[', '9', '9', ';', '6', 'u'}, cp: 99, mod: 6, ok: true},
+		{name: "ctrl+shift+c uppercase", seq: []byte{0x1b, '[', '6', '7', ';', '5', 'u'}, cp: 67, mod: 5, ok: true},
+		{name: "too short", seq: []byte{0x1b, '[', '9', '9'}, cp: 0, mod: 0, ok: false},
+		{name: "no escape prefix", seq: []byte{'[', '9', '9', ';', '6', 'u'}, cp: 0, mod: 0, ok: false},
+		{name: "no u terminator", seq: []byte{0x1b, '[', '9', '9', ';', '6', 'x'}, cp: 0, mod: 0, ok: false},
+		{name: "bad format no semicolon", seq: []byte{0x1b, '[', '9', '9', '6', 'u'}, cp: 0, mod: 0, ok: false},
+		{name: "empty", seq: []byte{}, cp: 0, mod: 0, ok: false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cp, mod, ok := parseCSIu(tt.seq)
+			if ok != tt.ok || cp != tt.cp || mod != tt.mod {
+				t.Fatalf("parseCSIu(%v) got=(%d,%d,%v) want=(%d,%d,%v)", tt.seq, cp, mod, ok, tt.cp, tt.mod, tt.ok)
+			}
+		})
+	}
+}
+
+func TestIsExitHotkeySeq(t *testing.T) {
+	cfg := &config.Config{HotkeyMod: "ctrl+alt"}
+
+	// CSI u Ctrl+Alt+C (mod=6)
+	if !isExitHotkeySeq([]byte{0x1b, '[', '9', '9', ';', '6', 'u'}, cfg) {
+		t.Fatalf("Ctrl+Alt+C CSI should exit with ctrl+alt config")
+	}
+	if !isExitHotkeySeq([]byte{0x1b, '[', '9', '9', ';', '7', 'u'}, cfg) {
+		t.Fatalf("Ctrl+Alt+Shift+C should also exit")
+	}
+	if isExitHotkeySeq([]byte{0x1b, '[', '9', '9', ';', '5', 'u'}, cfg) {
+		t.Fatalf("Ctrl+Shift+C should NOT exit with ctrl+alt config")
+	}
+	if isExitHotkeySeq([]byte{0x1b, '[', '9', '7', ';', '6', 'u'}, cfg) {
+		t.Fatalf("Ctrl+Alt+A should not exit")
+	}
+	if isExitHotkeySeq([]byte{0x1b, 'c'}, cfg) {
+		t.Fatalf("Alt+C (ESC c) should NOT exit — Ctrl modifier required")
+	}
+
+	cfg2 := &config.Config{HotkeyMod: "ctrl+shift"}
+	if !isExitHotkeySeq([]byte{0x1b, '[', '9', '9', ';', '5', 'u'}, cfg2) {
+		t.Fatalf("Ctrl+Shift+C should exit with ctrl+shift config")
+	}
+	if !isExitHotkeySeq([]byte{0x1b, '[', '9', '9', ';', '7', 'u'}, cfg2) {
+		t.Fatalf("Ctrl+Shift+Alt+C should also exit (includes Ctrl+Shift)")
+	}
+	if isExitHotkeySeq([]byte{0x1b, '[', '9', '9', ';', '6', 'u'}, cfg2) {
+		t.Fatalf("Ctrl+Alt+C should NOT exit with ctrl+shift config")
+	}
+	if isExitHotkeySeq([]byte{0x1b, 'c'}, cfg2) {
+		t.Fatalf("ESC c should NOT exit with ctrl+shift config")
+	}
+	if isExitHotkeySeq([]byte{0x1b, 'x'}, cfg2) {
+		t.Fatalf("ESC x should not exit")
+	}
+	if isExitHotkeySeq([]byte("hello"), cfg2) {
+		t.Fatalf("plain bytes should not exit")
+	}
+
+	cfg3 := &config.Config{HotkeyMod: "ctrl+alt"}
+	if isExitHotkeySeq([]byte{0x1b, '[', '9', '9', ';', '4', 'u'}, cfg3) {
+		t.Fatalf("Ctrl+C (without Alt) should not exit")
+	}
+	if isExitHotkeySeq([]byte{0x1b, '[', '9', '9', ';', '2', 'u'}, cfg3) {
+		t.Fatalf("Alt+C (without Ctrl) should not exit")
+	}
+}