test(calendar-sync): fix weekday-string lookup to match production

The boundary test for `calendar-sync--expand-weekly` with a 5-element UNTIL built its byday string from `'("SU" "MO" "TU" "WE" "TH" "FR" "SA")`, a 0-indexed Sunday-first array. The production code uses Monday=1, Sunday=7 throughout: `calendar-sync--date-weekday` returns it that way and `calendar-sync--weekday-to-number` expects it. When start-date landed on a Sunday (start-weekday=7), `(nth 7 array)` overran the 7-element list and returned nil. Then byday=(nil), and inside expand-weekly `(mod (- nil current-weekday) 7)` raised "wrong-type-argument number-or-marker-p nil".

The mismatch made the test fail every Saturday (when "tomorrow" is Sunday) and pass the other six days. The flake had been blamed on stale `.elc` in earlier triage, but `make clean && make test` reproduced the failure on Saturdays.

I switched the lookup to `(nth (1- start-weekday) '("MO" "TU" "WE" "TH" "FR" "SA" "SU"))`, the same convention as every other weekday-mapping in the codebase. I verified across all 7 weekdays via a faked `current-time`: each produces exactly 1 occurrence as expected.

Production code is internally consistent. No production change needed.

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