feat(quando-5ol): implement format presets and constants

Add Format type with preset constants (ISO, EU, US, Long, RFC2822) and
Format() method for convenient date formatting.

Key features:
- Type-safe Format enum with 5 preset formats
- ISO (2026-02-09), EU (09.02.2026), US (02/09/2026)
- Long format with i18n support (EN: "February 9, 2026", DE: "9. Februar 2026")
- RFC2822 email format
- Language-independent formats (ISO, EU, US, RFC2822) ignore Lang setting
- Language-dependent Long format respects WithLang()

Implementation:
- format.go: Format type, constants, Format() method, formatLong() helper
- format_test.go: 26 tests covering all formats, edge cases, immutability
- example_test.go: 8 example functions demonstrating usage

Performance (exceeds targets):
- ISO/EU/US: ~90 ns (target: <5 µs)
- RFC2822: ~207 ns (target: <5 µs)
- Long EN/DE: ~270 ns (target: <10 µs)

All tests pass with comprehensive coverage of leap years, month boundaries,
and language dependency.

example_test.go

@@ -629,3 +629,77 @@ func ExampleParseRelative_error() {
 	}
 	// Output: Complex expressions not yet supported
 }
+
+// ExampleDate_Format demonstrates basic date formatting
+func ExampleDate_Format() {
+	date := quando.From(time.Date(2026, 2, 9, 12, 30, 45, 0, time.UTC))
+
+	fmt.Println(date.Format(quando.ISO))
+	fmt.Println(date.Format(quando.EU))
+	fmt.Println(date.Format(quando.US))
+	fmt.Println(date.Format(quando.Long))
+	// Output:
+	// 2026-02-09
+	// 09.02.2026
+	// 02/09/2026
+	// February 9, 2026
+}
+
+// ExampleDate_Format_isoFormat demonstrates ISO 8601 format
+func ExampleDate_Format_isoFormat() {
+	date := quando.From(time.Date(2026, 2, 9, 0, 0, 0, 0, time.UTC))
+	fmt.Println(date.Format(quando.ISO))
+	// Output: 2026-02-09
+}
+
+// ExampleDate_Format_euFormat demonstrates European format
+func ExampleDate_Format_euFormat() {
+	date := quando.From(time.Date(2026, 2, 9, 0, 0, 0, 0, time.UTC))
+	fmt.Println(date.Format(quando.EU))
+	// Output: 09.02.2026
+}
+
+// ExampleDate_Format_usFormat demonstrates US format
+func ExampleDate_Format_usFormat() {
+	date := quando.From(time.Date(2026, 2, 9, 0, 0, 0, 0, time.UTC))
+	fmt.Println(date.Format(quando.US))
+	// Output: 02/09/2026
+}
+
+// ExampleDate_Format_longFormat demonstrates long format in English
+func ExampleDate_Format_longFormat() {
+	date := quando.From(time.Date(2026, 2, 9, 0, 0, 0, 0, time.UTC))
+	fmt.Println(date.Format(quando.Long))
+	// Output: February 9, 2026
+}
+
+// ExampleDate_Format_longFormatGerman demonstrates long format in German
+func ExampleDate_Format_longFormatGerman() {
+	date := quando.From(time.Date(2026, 2, 9, 0, 0, 0, 0, time.UTC)).WithLang(quando.DE)
+	fmt.Println(date.Format(quando.Long))
+	// Output: 9. Februar 2026
+}
+
+// ExampleDate_Format_rfc2822Format demonstrates RFC 2822 email format
+func ExampleDate_Format_rfc2822Format() {
+	date := quando.From(time.Date(2026, 2, 9, 12, 30, 45, 0, time.UTC))
+	fmt.Println(date.Format(quando.RFC2822))
+	// Output: Mon, 09 Feb 2026 12:30:45 +0000
+}
+
+// ExampleDate_Format_languageIndependence demonstrates that most formats ignore language
+func ExampleDate_Format_languageIndependence() {
+	date := quando.From(time.Date(2026, 2, 9, 0, 0, 0, 0, time.UTC))
+
+	// ISO format is always the same regardless of language
+	dateEN := date.WithLang(quando.EN)
+	dateDE := date.WithLang(quando.DE)
+
+	fmt.Println("ISO (EN):", dateEN.Format(quando.ISO))
+	fmt.Println("ISO (DE):", dateDE.Format(quando.ISO))
+	fmt.Println("Same:", dateEN.Format(quando.ISO) == dateDE.Format(quando.ISO))
+	// Output:
+	// ISO (EN): 2026-02-09
+	// ISO (DE): 2026-02-09
+	// Same: true
+}