feat(calendar-sync): add RECURRENCE-ID exception handling for recurring events

Handle rescheduled instances of recurring calendar events by processing
RECURRENCE-ID properties from ICS files. When someone reschedules a single
instance of a recurring meeting in Google Calendar, the calendar-sync module
now shows the rescheduled time instead of the original RRULE time.

New functions:
- calendar-sync--get-recurrence-id: Extract RECURRENCE-ID from event
- calendar-sync--get-recurrence-id-line: Get full line with TZID params
- calendar-sync--parse-recurrence-id: Parse into (year month day hour minute)
- calendar-sync--collect-recurrence-exceptions: Collect all exceptions by UID
- calendar-sync--occurrence-matches-exception-p: Match occurrences to exceptions
- calendar-sync--apply-single-exception: Apply exception data to occurrence
- calendar-sync--apply-recurrence-exceptions: Apply all exceptions to occurrences

Also adds DeepSat calendar configuration (dcal-file) to user-constants,
init.el, and org-agenda-config.

48 unit and integration tests added covering normal, boundary, and error cases.

1 files changed, 174 insertions, 0 deletions

diff --git a/tests/test-calendar-sync--collect-recurrence-exceptions.el b/tests/test-calendar-sync--collect-recurrence-exceptions.el
new file mode 100644
index 00000000..d2567b3a
--- /dev/null
+++ b/tests/test-calendar-sync--collect-recurrence-exceptions.el
@@ -0,0 +1,174 @@
+;;; test-calendar-sync--collect-recurrence-exceptions.el --- Tests for exception collection  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for calendar-sync--collect-recurrence-exceptions function.
+;; Tests collecting all RECURRENCE-ID events from ICS content.
+;; Following quality-engineer.org guidelines: one function per file.
+
+;;; Code:
+
+(require 'ert)
+(add-to-list 'load-path (expand-file-name "." (file-name-directory load-file-name)))
+(add-to-list 'load-path (expand-file-name "../modules" (file-name-directory load-file-name)))
+(require 'testutil-calendar-sync)
+(require 'calendar-sync)
+
+;;; Test Data Helpers
+
+(defun test-make-exception-event (uid recurrence-id summary start end)
+  "Create a VEVENT with RECURRENCE-ID for testing.
+UID is the base event UID. RECURRENCE-ID is the original occurrence date.
+SUMMARY, START, END define the exception event."
+  (concat "BEGIN:VEVENT\n"
+          "UID:" uid "\n"
+          "RECURRENCE-ID:" recurrence-id "\n"
+          "SUMMARY:" summary "\n"
+          "DTSTART:" (test-calendar-sync-ics-datetime start) "\n"
+          "DTEND:" (test-calendar-sync-ics-datetime end) "\n"
+          "END:VEVENT"))
+
+(defun test-make-base-event-with-rrule (uid summary start end rrule)
+  "Create a recurring VEVENT with RRULE for testing."
+  (concat "BEGIN:VEVENT\n"
+          "UID:" uid "\n"
+          "SUMMARY:" summary "\n"
+          "DTSTART:" (test-calendar-sync-ics-datetime start) "\n"
+          "DTEND:" (test-calendar-sync-ics-datetime end) "\n"
+          "RRULE:" rrule "\n"
+          "END:VEVENT"))
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--collect-recurrence-exceptions-normal-single-exception ()
+  "Test collecting single exception event."
+  (let* ((start (test-calendar-sync-time-days-from-now 7 10 0))
+         (end (test-calendar-sync-time-days-from-now 7 11 0))
+         (ics (test-calendar-sync-make-ics
+               (test-make-exception-event "event123@google.com"
+                                          "20260203T090000Z"
+                                          "Rescheduled Meeting"
+                                          start end)))
+         (exceptions (calendar-sync--collect-recurrence-exceptions ics)))
+    ;; Should have one exception keyed by UID
+    (should (hash-table-p exceptions))
+    (should (gethash "event123@google.com" exceptions))))
+
+(ert-deftest test-calendar-sync--collect-recurrence-exceptions-normal-multiple-same-uid ()
+  "Test collecting multiple exceptions for same recurring event."
+  (let* ((start1 (test-calendar-sync-time-days-from-now 7 10 0))
+         (end1 (test-calendar-sync-time-days-from-now 7 11 0))
+         (start2 (test-calendar-sync-time-days-from-now 14 11 0))
+         (end2 (test-calendar-sync-time-days-from-now 14 12 0))
+         (ics (test-calendar-sync-make-ics
+               (test-make-exception-event "weekly123@google.com"
+                                          "20260203T090000Z"
+                                          "Week 1 Rescheduled"
+                                          start1 end1)
+               (test-make-exception-event "weekly123@google.com"
+                                          "20260210T090000Z"
+                                          "Week 2 Rescheduled"
+                                          start2 end2)))
+         (exceptions (calendar-sync--collect-recurrence-exceptions ics)))
+    ;; Should have list of exceptions under single UID
+    (should (hash-table-p exceptions))
+    (let ((uid-exceptions (gethash "weekly123@google.com" exceptions)))
+      (should uid-exceptions)
+      (should (= 2 (length uid-exceptions))))))
+
+(ert-deftest test-calendar-sync--collect-recurrence-exceptions-normal-multiple-uids ()
+  "Test collecting exceptions for different recurring events."
+  (let* ((start (test-calendar-sync-time-days-from-now 7 10 0))
+         (end (test-calendar-sync-time-days-from-now 7 11 0))
+         (ics (test-calendar-sync-make-ics
+               (test-make-exception-event "event-a@google.com"
+                                          "20260203T090000Z"
+                                          "Event A Rescheduled"
+                                          start end)
+               (test-make-exception-event "event-b@google.com"
+                                          "20260203T140000Z"
+                                          "Event B Rescheduled"
+                                          start end)))
+         (exceptions (calendar-sync--collect-recurrence-exceptions ics)))
+    ;; Should have two UIDs
+    (should (hash-table-p exceptions))
+    (should (gethash "event-a@google.com" exceptions))
+    (should (gethash "event-b@google.com" exceptions))))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--collect-recurrence-exceptions-boundary-no-exceptions ()
+  "Test ICS with no RECURRENCE-ID events returns empty hash."
+  (let* ((start (test-calendar-sync-time-days-from-now 7 10 0))
+         (end (test-calendar-sync-time-days-from-now 7 11 0))
+         (ics (test-calendar-sync-make-ics
+               (test-calendar-sync-make-vevent "Regular Event" start end)))
+         (exceptions (calendar-sync--collect-recurrence-exceptions ics)))
+    (should (hash-table-p exceptions))
+    (should (= 0 (hash-table-count exceptions)))))
+
+(ert-deftest test-calendar-sync--collect-recurrence-exceptions-boundary-mixed-events ()
+  "Test ICS with both regular and exception events."
+  (let* ((start (test-calendar-sync-time-days-from-now 7 10 0))
+         (end (test-calendar-sync-time-days-from-now 7 11 0))
+         (base-start (test-calendar-sync-time-days-from-now 1 9 0))
+         (base-end (test-calendar-sync-time-days-from-now 1 10 0))
+         (ics (test-calendar-sync-make-ics
+               ;; Regular event (no RECURRENCE-ID)
+               (test-calendar-sync-make-vevent "Normal Meeting" base-start base-end)
+               ;; Base recurring event with RRULE
+               (test-make-base-event-with-rrule "recurring@google.com"
+                                                 "Weekly Sync"
+                                                 base-start base-end
+                                                 "FREQ=WEEKLY;COUNT=10")
+               ;; Exception to the recurring event
+               (test-make-exception-event "recurring@google.com"
+                                          "20260210T090000Z"
+                                          "Weekly Sync (Rescheduled)"
+                                          start end)))
+         (exceptions (calendar-sync--collect-recurrence-exceptions ics)))
+    ;; Should only collect the exception, not regular or base events
+    (should (hash-table-p exceptions))
+    (should (= 1 (hash-table-count exceptions)))
+    (should (gethash "recurring@google.com" exceptions))))
+
+(ert-deftest test-calendar-sync--collect-recurrence-exceptions-boundary-tzid-exception ()
+  "Test exception event with TZID-qualified RECURRENCE-ID."
+  (let* ((start (test-calendar-sync-time-days-from-now 7 10 0))
+         (end (test-calendar-sync-time-days-from-now 7 11 0))
+         (event (concat "BEGIN:VEVENT\n"
+                        "UID:tzid-event@google.com\n"
+                        "RECURRENCE-ID;TZID=Europe/Tallinn:20260203T170000\n"
+                        "SUMMARY:Tallinn Meeting Rescheduled\n"
+                        "DTSTART:" (test-calendar-sync-ics-datetime start) "\n"
+                        "DTEND:" (test-calendar-sync-ics-datetime end) "\n"
+                        "END:VEVENT"))
+         (ics (test-calendar-sync-make-ics event))
+         (exceptions (calendar-sync--collect-recurrence-exceptions ics)))
+    (should (hash-table-p exceptions))
+    (should (gethash "tzid-event@google.com" exceptions))))
+
+;;; Error Cases
+
+(ert-deftest test-calendar-sync--collect-recurrence-exceptions-error-empty-string ()
+  "Test empty ICS content returns empty hash."
+  (let ((exceptions (calendar-sync--collect-recurrence-exceptions "")))
+    (should (hash-table-p exceptions))
+    (should (= 0 (hash-table-count exceptions)))))
+
+(ert-deftest test-calendar-sync--collect-recurrence-exceptions-error-nil-input ()
+  "Test nil input returns empty hash or nil."
+  (let ((exceptions (calendar-sync--collect-recurrence-exceptions nil)))
+    (should (or (null exceptions)
+                (and (hash-table-p exceptions)
+                     (= 0 (hash-table-count exceptions)))))))
+
+(ert-deftest test-calendar-sync--collect-recurrence-exceptions-error-malformed-ics ()
+  "Test malformed ICS handles gracefully."
+  (let ((exceptions (calendar-sync--collect-recurrence-exceptions "not valid ics content")))
+    ;; Should not crash, return empty hash
+    (should (or (null exceptions)
+                (and (hash-table-p exceptions)
+                     (= 0 (hash-table-count exceptions)))))))
+
+(provide 'test-calendar-sync--collect-recurrence-exceptions)
+;;; test-calendar-sync--collect-recurrence-exceptions.el ends here