fix: prevent server crash on unsupported Shift-JIS characters (#116)

UTF8ToSJIS panicked when encountering characters outside the Shift-JIS
range (emoji, Lenny faces, cuneiform, etc.), crashing the server when
such characters were sent via the Discord relay channel.

Replace the panic with graceful filtering that drops unmappable runes
and preserves valid content. Also fix ToNGWord index-out-of-range panic
on empty encoder output.

Closes #116

common/stringsupport/string_convert_test.go

@@ -458,6 +458,80 @@ func BenchmarkCSVElems(b *testing.B) {
 	}
 }
 
+func TestUTF8ToSJIS_UnsupportedCharacters(t *testing.T) {
+	// Regression test for PR #116: Characters outside the Shift-JIS range
+	// (e.g. Lenny face, cuneiform) previously caused a panic in UTF8ToSJIS,
+	// crashing the server when relayed from Discord.
+	tests := []struct {
+		name  string
+		input string
+	}{
+		{"lenny_face", "( ͡° ͜ʖ ͡°)"},
+		{"cuneiform", "𒀜"},
+		{"emoji", "Hello 🎮 World"},
+		{"mixed_unsupported", "Test ͡° message 𒀜 here"},
+		{"zalgo_text", "H̷e̸l̵l̶o̷"},
+		{"only_unsupported", "🎮🎲🎯"},
+		{"cyrillic", "Привет"},
+		{"arabic", "مرحبا"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Must not panic - the old code would panic here
+			defer func() {
+				if r := recover(); r != nil {
+					t.Errorf("UTF8ToSJIS panicked on input %q: %v", tt.input, r)
+				}
+			}()
+			result := UTF8ToSJIS(tt.input)
+			if result == nil {
+				t.Error("UTF8ToSJIS returned nil")
+			}
+		})
+	}
+}
+
+func TestUTF8ToSJIS_PreservesValidContent(t *testing.T) {
+	// Verify that valid Shift-JIS content is preserved when mixed with
+	// unsupported characters.
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{"ascii_with_emoji", "Hello 🎮 World", "Hello  World"},
+		{"japanese_with_emoji", "テスト🎮データ", "テストデータ"},
+		{"only_valid", "Hello World", "Hello World"},
+		{"only_invalid", "🎮🎲🎯", ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			sjis := UTF8ToSJIS(tt.input)
+			roundTripped := SJISToUTF8(sjis)
+			if roundTripped != tt.expected {
+				t.Errorf("UTF8ToSJIS(%q) round-tripped to %q, want %q", tt.input, roundTripped, tt.expected)
+			}
+		})
+	}
+}
+
+func TestToNGWord_UnsupportedCharacters(t *testing.T) {
+	// ToNGWord also calls UTF8ToSJIS internally, so it must not panic either.
+	inputs := []string{"( ͡° ͜ʖ ͡°)", "🎮", "Hello 🎮 World"}
+	for _, input := range inputs {
+		t.Run(input, func(t *testing.T) {
+			defer func() {
+				if r := recover(); r != nil {
+					t.Errorf("ToNGWord panicked on input %q: %v", input, r)
+				}
+			}()
+			_ = ToNGWord(input)
+		})
+	}
+}
+
 func BenchmarkUTF8ToSJIS(b *testing.B) {
 	text := "Hello World テスト"
 	b.ResetTimer()