fix(share): keep the merge char when it is the only changed setting

The compact wire wrote `mg` only alongside another non-default setting,
so a link where only the join character changed decoded back to `+`.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: dani-polani

bitext/src/lib/serialization/compact-v4.ts

@@ -75,11 +75,11 @@ function settingsToCompact(rounded: VisualSettingsV2): CompactSettings4 | undefi
 	if (rounded.background !== def.background) {
 		o.bg = rounded.background === 'dark' ? 1 : 0;
 	}
-	const keysBeforeFinalize = Object.keys(o).length;
-	if (keysBeforeFinalize > 0) {
+	// Decoding treats a present `s` without `mg` as "no merge char", so any emitted `s` carries it.
+	if (Object.keys(o).length > 0 || rounded.tokenMergeChar !== def.tokenMergeChar) {
 		o.mg = rounded.tokenMergeChar;
 	}
-	return keysBeforeFinalize > 0 ? sortKeys(o) : undefined;
+	return Object.keys(o).length > 0 ? sortKeys(o) : undefined;
 }
 
 function compactToVisualSettings(s: CompactSettings4 | undefined): VisualSettingsV2 {

bitext/src/lib/serialization/serialization-roundtrip.test.ts

@@ -284,6 +284,14 @@ describe('compact v4 encode/decode (current share format)', () => {
 		expect(decoded.settings.tokenSplitChars).toBe('.');
 	});
 
+	it('round-trip: a changed merge char survives when it is the only non-default setting', () => {
+		const base = migrate({});
+		for (const tokenMergeChar of ['_', '']) {
+			const s: AppStateV2 = { ...base, settings: { ...base.settings, tokenMergeChar } };
+			expect(decodeState(encodeState(s)).settings.tokenMergeChar).toBe(tokenMergeChar);
+		}
+	});
+
 	it('round-trip: token link color mode (background)', () => {
 		const base = migrate({});
 		const s: AppStateV2 = {