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, 166 insertions, 0 deletions

diff --git a/tests/test-integration-calendar-sync-recurrence-exceptions.el b/tests/test-integration-calendar-sync-recurrence-exceptions.el
new file mode 100644
index 00000000..0a9b5af1
--- /dev/null
+++ b/tests/test-integration-calendar-sync-recurrence-exceptions.el
@@ -0,0 +1,166 @@
+;;; test-integration-calendar-sync-recurrence-exceptions.el --- Integration tests  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Integration tests for RECURRENCE-ID exception handling workflow.
+;; Tests the complete pipeline: ICS parsing → RRULE expansion → exception application.
+;; Tests verify the final org output contains correct relative times.
+
+;;; 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 - Using local times (no Z suffix) to avoid timezone conversion
+
+(defun test-integration-make-recurring-event-with-exception-local ()
+  "Create ICS content with local times (no Z suffix).
+Weekly meeting with one instance rescheduled from 09:00 to 10:00."
+  (let* ((base-start (test-calendar-sync-time-days-from-now 0 9 0))
+         (base-end (test-calendar-sync-time-days-from-now 0 10 0))
+         ;; Exception: week 2 moved from 9:00 to 10:00
+         (exception-start (test-calendar-sync-time-days-from-now 7 10 0))
+         (exception-end (test-calendar-sync-time-days-from-now 7 11 0))
+         (recurrence-id-date (test-calendar-sync-time-days-from-now 7 9 0)))
+    (concat "BEGIN:VCALENDAR\n"
+            "VERSION:2.0\n"
+            "PRODID:-//Test//Test//EN\n"
+            ;; Base recurring event - use local time (no Z)
+            "BEGIN:VEVENT\n"
+            "UID:weekly-meeting@google.com\n"
+            "SUMMARY:Weekly Team Sync\n"
+            "DTSTART:" (test-calendar-sync-ics-datetime-local base-start) "\n"
+            "DTEND:" (test-calendar-sync-ics-datetime-local base-end) "\n"
+            "RRULE:FREQ=WEEKLY;COUNT=4\n"
+            "END:VEVENT\n"
+            ;; Exception event - local time
+            "BEGIN:VEVENT\n"
+            "UID:weekly-meeting@google.com\n"
+            "RECURRENCE-ID:" (test-calendar-sync-ics-datetime-local recurrence-id-date) "\n"
+            "SUMMARY:Weekly Team Sync (Rescheduled)\n"
+            "DTSTART:" (test-calendar-sync-ics-datetime-local exception-start) "\n"
+            "DTEND:" (test-calendar-sync-ics-datetime-local exception-end) "\n"
+            "END:VEVENT\n"
+            "END:VCALENDAR")))
+
+;;; Integration Tests
+
+(ert-deftest test-integration-recurrence-exception-org-output ()
+  "Test that parse-ics returns valid org content with exception summary."
+  (let* ((ics-content (test-integration-make-recurring-event-with-exception-local))
+         (org-output (calendar-sync--parse-ics ics-content)))
+    ;; Should return string with org content
+    (should (stringp org-output))
+    (should (string-match-p "Weekly Team Sync" org-output))
+    ;; Should contain the rescheduled event with correct summary
+    (should (string-match-p "Rescheduled" org-output))))
+
+(ert-deftest test-integration-recurrence-exception-times-in-output ()
+  "Test that rescheduled event shows different time than regular occurrences."
+  (let* ((ics-content (test-integration-make-recurring-event-with-exception-local))
+         (org-output (calendar-sync--parse-ics ics-content)))
+    ;; The org output should contain both 09:00 (regular) and 10:00 (rescheduled)
+    (should (string-match-p "09:00" org-output))
+    (should (string-match-p "10:00" org-output))))
+
+(ert-deftest test-integration-recurrence-no-exceptions-org-output ()
+  "Test recurring event without exceptions produces correct org output."
+  (let* ((base-start (test-calendar-sync-time-days-from-now 0 9 0))
+         (base-end (test-calendar-sync-time-days-from-now 0 10 0))
+         (ics-content (concat "BEGIN:VCALENDAR\n"
+                              "VERSION:2.0\n"
+                              "BEGIN:VEVENT\n"
+                              "UID:simple-weekly@google.com\n"
+                              "SUMMARY:Simple Weekly\n"
+                              "DTSTART:" (test-calendar-sync-ics-datetime-local base-start) "\n"
+                              "DTEND:" (test-calendar-sync-ics-datetime-local base-end) "\n"
+                              "RRULE:FREQ=WEEKLY;COUNT=3\n"
+                              "END:VEVENT\n"
+                              "END:VCALENDAR"))
+         (org-output (calendar-sync--parse-ics ics-content)))
+    ;; Should have org content
+    (should (stringp org-output))
+    ;; Should have the event title
+    (should (string-match-p "Simple Weekly" org-output))
+    ;; All occurrences should be at 09:00 (no exceptions)
+    (should (string-match-p "09:00-10:00" org-output))))
+
+(ert-deftest test-integration-collect-exceptions-from-ics ()
+  "Test that collect-recurrence-exceptions correctly extracts exceptions."
+  (let* ((ics-content (test-integration-make-recurring-event-with-exception-local))
+         (exceptions (calendar-sync--collect-recurrence-exceptions ics-content)))
+    ;; Should have collected one exception
+    (should (hash-table-p exceptions))
+    (should (gethash "weekly-meeting@google.com" exceptions))
+    ;; The exception should exist
+    (let ((exc-list (gethash "weekly-meeting@google.com" exceptions)))
+      (should (= 1 (length exc-list)))
+      ;; The exception should have the rescheduled time (10:00)
+      (let* ((exc (car exc-list))
+             (exc-start (plist-get exc :start)))
+        (should (= 10 (nth 3 exc-start)))))))
+
+(ert-deftest test-integration-multiple-events-with-exceptions ()
+  "Test multiple recurring events each with their own exceptions."
+  (let* ((start-a (test-calendar-sync-time-days-from-now 0 9 0))
+         (end-a (test-calendar-sync-time-days-from-now 0 10 0))
+         (start-b (test-calendar-sync-time-days-from-now 0 14 0))
+         (end-b (test-calendar-sync-time-days-from-now 0 15 0))
+         ;; Exceptions for week 2: A moves from 9:00→10:00, B moves from 14:00→15:00
+         (exc-a-start (test-calendar-sync-time-days-from-now 7 10 0))
+         (exc-a-end (test-calendar-sync-time-days-from-now 7 11 0))
+         (exc-b-start (test-calendar-sync-time-days-from-now 7 15 0))
+         (exc-b-end (test-calendar-sync-time-days-from-now 7 16 0))
+         (rec-id-a (test-calendar-sync-time-days-from-now 7 9 0))
+         (rec-id-b (test-calendar-sync-time-days-from-now 7 14 0))
+         (ics-content
+          (concat "BEGIN:VCALENDAR\n"
+                  "VERSION:2.0\n"
+                  ;; Event A base
+                  "BEGIN:VEVENT\n"
+                  "UID:event-a@google.com\n"
+                  "SUMMARY:Morning Standup\n"
+                  "DTSTART:" (test-calendar-sync-ics-datetime-local start-a) "\n"
+                  "DTEND:" (test-calendar-sync-ics-datetime-local end-a) "\n"
+                  "RRULE:FREQ=WEEKLY;COUNT=2\n"
+                  "END:VEVENT\n"
+                  ;; Event A exception
+                  "BEGIN:VEVENT\n"
+                  "UID:event-a@google.com\n"
+                  "RECURRENCE-ID:" (test-calendar-sync-ics-datetime-local rec-id-a) "\n"
+                  "SUMMARY:Morning Standup (Moved)\n"
+                  "DTSTART:" (test-calendar-sync-ics-datetime-local exc-a-start) "\n"
+                  "DTEND:" (test-calendar-sync-ics-datetime-local exc-a-end) "\n"
+                  "END:VEVENT\n"
+                  ;; Event B base
+                  "BEGIN:VEVENT\n"
+                  "UID:event-b@google.com\n"
+                  "SUMMARY:Afternoon Review\n"
+                  "DTSTART:" (test-calendar-sync-ics-datetime-local start-b) "\n"
+                  "DTEND:" (test-calendar-sync-ics-datetime-local end-b) "\n"
+                  "RRULE:FREQ=WEEKLY;COUNT=2\n"
+                  "END:VEVENT\n"
+                  ;; Event B exception
+                  "BEGIN:VEVENT\n"
+                  "UID:event-b@google.com\n"
+                  "RECURRENCE-ID:" (test-calendar-sync-ics-datetime-local rec-id-b) "\n"
+                  "SUMMARY:Afternoon Review (Moved)\n"
+                  "DTSTART:" (test-calendar-sync-ics-datetime-local exc-b-start) "\n"
+                  "DTEND:" (test-calendar-sync-ics-datetime-local exc-b-end) "\n"
+                  "END:VEVENT\n"
+                  "END:VCALENDAR"))
+         (org-output (calendar-sync--parse-ics ics-content)))
+    (should (stringp org-output))
+    ;; Should have both events
+    (should (string-match-p "Morning Standup" org-output))
+    (should (string-match-p "Afternoon Review" org-output))
+    ;; Should have the "(Moved)" versions
+    (should (string-match-p "Moved" org-output))
+    ;; Should have rescheduled times (10:00 and 15:00)
+    (should (string-match-p "10:00" org-output))
+    (should (string-match-p "15:00" org-output))))
+
+(provide 'test-integration-calendar-sync-recurrence-exceptions)
+;;; test-integration-calendar-sync-recurrence-exceptions.el ends here