Fix concurrency safety, panic prevention, and weekday range bug

[Copilot]

Comprehensive audit identified data races, panic-prone APIs, and incorrect weekday filtering logic.

Concurrency Safety

• Added GetWeekStartDay() / SetWeekStartDay() with sync.RWMutex protection
• Deprecated direct access to WeekStartDay global (retained for backward compatibility)

Panic Prevention

• Added *Safe variants for all weekday methods (MondaySafe, TuesdaySafe, etc.) that return errors instead of panicking
• Extracted weekdaySafe() helper to reduce duplication

Nil Safety

• Added getWeekStartDay(), getTimeFormats(), getTimeLocation() helpers for nil-safe Rule access in Timex

Bug Fix

• Fixed GetWeekdaysInRange() which had broken leap year logic that skipped valid weekdays

// Before: weekdays incorrectly filtered in non-leap years
func GetWeekdaysInRange(start, end time.Time) []time.Time {
    // ... flawed IsLeapYear check that skipped days
}

// After: simple weekday check
func GetWeekdaysInRange(start, end time.Time) []time.Time {
    for current := start; current.Before(end) || current.Equal(end); current = current.AddDate(0, 0, 1) {
        if d := current.Weekday(); d != time.Sunday && d != time.Saturday {
            weekdays = append(weekdays, current)
        }
    }
}

Usage

// Thread-safe week start configuration
timefy.SetWeekStartDay(time.Monday)
day := timefy.GetWeekStartDay()

// Error-returning weekday methods
monday, err := tmx.MondaySafe("2023-10-25")
if err != nil {
    // handle parse error
}