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.

1 files changed, 8 insertions, 1 deletions

diff --git a/tests/test-calendar-sync--date-to-time.el b/tests/test-calendar-sync--date-to-time.el
index 931d5929..b877eb06 100644
--- a/tests/test-calendar-sync--date-to-time.el
+++ b/tests/test-calendar-sync--date-to-time.el
@@ -104,7 +104,14 @@ Mirrors the real DeepSat Mgmt Sync case: RRULE with UNTIL ~6 days after start."
          (until-5 (append (test-calendar-sync-time-date-only 7) '(19 59)))
          (start-weekday (calendar-sync--date-weekday
                          (list (nth 0 start-date) (nth 1 start-date) (nth 2 start-date))))
-         (weekday-str (nth start-weekday '("SU" "MO" "TU" "WE" "TH" "FR" "SA")))
+         ;; calendar-sync--date-weekday returns 1..7 with Monday=1, Sunday=7
+         ;; (matching calendar-sync--weekday-to-number), so the lookup array
+         ;; runs Mon..Sun and we index with (1- start-weekday). The earlier
+         ;; "SU MO TU WE TH FR SA" array misread the convention as 0-indexed
+         ;; Sunday-first, which silently returned nil for start-weekday=7
+         ;; (Sunday) and crashed inside expand-weekly every Saturday.
+         (weekday-str (nth (1- start-weekday)
+                           '("MO" "TU" "WE" "TH" "FR" "SA" "SU")))
          (base-event (list :summary "Short-Lived Series"
                            :start start-date
                            :end end-date))