clock: add weeks-numbering option for configurable CW calculation

[beyertom]

Summary

• The iso8601 flag previously coupled calendar grid layout with week number
  format as a single boolean, giving no way to choose e.g. ISO week numbering
  independently of the locale's first-day-of-week setting
• Adds a new calendar.weeks-numbering option (iso / monday / sunday)
  that explicitly controls only the week number calculation method
• Existing behaviour is fully preserved when the option is not set

Usage

"calendar": {
  "weeks-pos": "right",
  "weeks-numbering": "iso"
}

Test plan

• weeks-numbering not set → same behaviour as before (iso8601/locale driven)
• weeks-numbering: "iso" → uses {:%V}, ISO 8601 week numbers
• weeks-numbering: "monday" → uses {:%W}, Monday-based week numbers
• weeks-numbering: "sunday" → uses {:%U}, Sunday-based week numbers
• Invalid value logs a warning and falls back to locale default
• Man page renders correctly (scdoc waybar-clock.5.scd)