feat(quando-95w): implement custom layout formatting with i18n support

Add FormatLayout() method enabling custom date formatting using Go's
standard layout format (Mon Jan 2 15:04:05 MST 2006) with full
internationalization support for month and weekday names.

Key features:
- Custom layout strings using Go's time.Format() reference date
- i18n support for EN and DE languages
- Translates both full and abbreviated month/weekday names
- Fast path for EN (direct passthrough, ~144-173 ns/op)
- Optimized replacements using strings.NewReplacer (~7.3-7.5 µs/op for DE)
- Comprehensive test coverage (97.6%)

Implementation:
- Added FormatLayout() method to format.go
- Uses strings.NewReplacer for atomic replacements to avoid substring collisions
- Added 280+ lines of tests covering all months, weekdays, and edge cases
- Added 4 benchmark tests (all meet <10µs target)
- Added 4 example tests demonstrating usage

Performance:
- EN (fast path): ~144-173 ns/op
- DE (with i18n): ~7.3-7.5 µs/op
- Both well under <10µs requirement

Files changed:
- format.go: Added FormatLayout() method with full godoc
- format_test.go: Added comprehensive tests and benchmarks
- example_test.go: Added 4 example functions

format.go

@@ -2,6 +2,7 @@ package quando
 
 import (
 	"fmt"
+	"strings"
 	"time"
 )
 
@@ -132,6 +133,92 @@ func (d Date) formatLong() string {
 	}
 }
 
+// FormatLayout formats the date using a custom layout string with localized month/weekday names.
+//
+// The layout parameter uses Go's standard time layout format (Mon Jan 2 15:04:05 MST 2006).
+// Month and weekday names in the output are translated according to the Date's Lang setting.
+//
+// Supported layout components (see time.Format documentation for full details):
+//   - "January" - Full month name (localized)
+//   - "Jan" - Short month name (localized)
+//   - "Monday" - Full weekday name (localized)
+//   - "Mon" - Short weekday name (localized)
+//   - All numeric components (year, day, hour, etc.) - not localized
+//
+// Language Support:
+//   - EN (English) - Default, no translation
+//   - DE (German) - Translates month/weekday names
+//
+// Performance: < 10 µs for typical layouts with i18n
+//
+// Example:
+//
+//	date := quando.From(time.Date(2026, 2, 9, 14, 30, 0, 0, time.UTC))
+//
+//	// English (default)
+//	date.FormatLayout("Monday, 2. January 2006")  // "Monday, 9. February 2026"
+//
+//	// German
+//	date.WithLang(quando.DE).FormatLayout("Monday, 2. January 2006")  // "Montag, 9. Februar 2026"
+//	date.WithLang(quando.DE).FormatLayout("Mon, 02 Jan 2006")         // "Mo, 09 Feb 2026"
+func (d Date) FormatLayout(layout string) string {
+	// Fast path: if lang is EN (or not set), just use Go's format directly
+	lang := d.lang
+	if lang == "" || lang == EN {
+		return d.t.Format(layout)
+	}
+
+	// Format using Go's time.Format (always in English)
+	formatted := d.t.Format(layout)
+
+	// Build replacement pairs: old (English) -> new (localized)
+	// Order matters: longest strings first to avoid partial matches
+	// We use strings.Replacer which processes all replacements in a single pass
+	var replacementPairs []string
+
+	// 1. Full month names first (e.g., "September" before "Sep")
+	for m := time.January; m <= time.December; m++ {
+		enFull := monthNames[EN][m-1]
+		localFull := lang.MonthName(m)
+		if enFull != localFull {
+			replacementPairs = append(replacementPairs, enFull, localFull)
+		}
+	}
+
+	// 2. Full weekday names (e.g., "Wednesday" before "Wed")
+	for wd := time.Sunday; wd <= time.Saturday; wd++ {
+		enFull := weekdayNames[EN][wd]
+		localFull := lang.WeekdayName(wd)
+		if enFull != localFull {
+			replacementPairs = append(replacementPairs, enFull, localFull)
+		}
+	}
+
+	// 3. Short month names
+	for m := time.January; m <= time.December; m++ {
+		enShort := monthNamesShort[EN][m-1]
+		localShort := lang.MonthNameShort(m)
+		if enShort != localShort {
+			replacementPairs = append(replacementPairs, enShort, localShort)
+		}
+	}
+
+	// 4. Short weekday names
+	for wd := time.Sunday; wd <= time.Saturday; wd++ {
+		enShort := weekdayNamesShort[EN][wd]
+		localShort := lang.WeekdayNameShort(wd)
+		if enShort != localShort {
+			replacementPairs = append(replacementPairs, enShort, localShort)
+		}
+	}
+
+	// Create a replacer and apply all replacements in a single pass
+	// This ensures that once a full name is replaced, the short name in the
+	// replacement won't be affected (e.g., "Monday" -> "Montag", and "Mon" in "Montag" won't become "Mo")
+	replacer := strings.NewReplacer(replacementPairs...)
+	return replacer.Replace(formatted)
+}
+
 // String returns the string representation of the Format type.
 // This is used for better test output and debugging.
 func (f Format) String() string {