feat(quando-41g): implement timezone conversion with DST support

Add In() method to Date type for converting between IANA timezones.
Implements comprehensive DST handling with proper wall-clock time
preservation across daylight saving transitions.

Features:
- In(location) converts to specified IANA timezone
- Returns ErrInvalidTimezone for invalid timezone names
- Never panics on invalid input
- Preserves language settings across conversions
- Maintains immutability pattern

DST Handling:
- Add(1, Days) preserves wall clock time, not duration
- Tested across spring forward (Mar 29, 2026)
- Tested across fall back (Oct 25, 2026)

Testing:
- 100% coverage for In() method
- 6 comprehensive test functions (228 lines)
- Tests for Europe/Berlin, America/New_York, Asia/Tokyo, UTC
- Error handling tests (empty string, invalid timezones)
- Immutability and language preservation tests
- 3 example tests demonstrating usage

Overall coverage: 98.1%

example_test.go

@@ -380,9 +380,9 @@ func ExampleParse() {
 // ExampleParse_formats demonstrates parsing various supported date formats
 func ExampleParse_formats() {
 	formats := []string{
-		"2026-02-09",  // ISO format (YYYY-MM-DD)
-		"2026/02/09",  // ISO with slash (YYYY/MM/DD)
-		"09.02.2026",  // EU format (DD.MM.YYYY)
+		"2026-02-09", // ISO format (YYYY-MM-DD)
+		"2026/02/09", // ISO with slash (YYYY/MM/DD)
+		"09.02.2026", // EU format (DD.MM.YYYY)
 	}
 
 	for _, f := range formats {
@@ -410,3 +410,51 @@ func ExampleParse_error() {
 	}
 	// Output: Ambiguous format detected
 }
+
+// ExampleDate_In demonstrates timezone conversion
+func ExampleDate_In() {
+	// Create a UTC time
+	utc := quando.From(time.Date(2026, 6, 15, 12, 0, 0, 0, time.UTC))
+
+	// Convert to Berlin timezone (UTC+2 in summer)
+	berlin, err := utc.In("Europe/Berlin")
+	if err != nil {
+		fmt.Println("Error:", err)
+		return
+	}
+
+	fmt.Printf("UTC:    %v\n", utc)
+	fmt.Printf("Berlin: %v\n", berlin)
+	// Output:
+	// UTC:    2026-06-15 12:00:00
+	// Berlin: 2026-06-15 14:00:00
+}
+
+// ExampleDate_In_dst demonstrates DST handling
+func ExampleDate_In_dst() {
+	// Winter: Europe/Berlin is UTC+1 (CET)
+	winter := quando.From(time.Date(2026, 1, 15, 12, 0, 0, 0, time.UTC))
+	berlin, _ := winter.In("Europe/Berlin")
+	fmt.Printf("Winter: UTC 12:00 -> Berlin %02d:00\n", berlin.Time().Hour())
+
+	// Summer: Europe/Berlin is UTC+2 (CEST)
+	summer := quando.From(time.Date(2026, 7, 15, 12, 0, 0, 0, time.UTC))
+	berlin, _ = summer.In("Europe/Berlin")
+	fmt.Printf("Summer: UTC 12:00 -> Berlin %02d:00\n", berlin.Time().Hour())
+
+	// Output:
+	// Winter: UTC 12:00 -> Berlin 13:00
+	// Summer: UTC 12:00 -> Berlin 14:00
+}
+
+// ExampleDate_In_error demonstrates error handling
+func ExampleDate_In_error() {
+	date := quando.Now()
+
+	_, err := date.In("Invalid/Timezone")
+	if errors.Is(err, quando.ErrInvalidTimezone) {
+		fmt.Println("Invalid timezone name")
+	}
+
+	// Output: Invalid timezone name
+}