test: cover pure-logic gaps found by the coverage audit

I ran make coverage and worked the report function by function, separating real gaps from interactive/IO wrappers that aren't unit-test targets. These tests fill the genuine pure-logic holes: predicates, parsers, formatters, transforms, and three modules that had no test file at all.

New files cover car-member (local-repository), show-kill-insert-item (show-kill-ring), the oauth2-auto plstore cache fix (auth-config), the coverage-core project-root fallback, reconcile--dirty-p, and the recurrence-frequency dispatch in calendar-sync. Extended files add the missing branches: coverage-core's merge-base and diff /dev/null handling plus the staged and branch-vs-main scopes, the detect-system-timezone symlink path, user-constants no-op and optional-failure branches, the elfeed playlist branch with HTML-entity decoding, the duplicate-line no-comment-syntax guard, and several calendar-sync edges (exception field overrides, timestamp seconds and TZID fallback, property-line position advancement, parse-ics nil and out-of-range inputs).

Mocks sit at the real boundaries (plstore, url-retrieve, process-file, git) so each function's own logic runs. Dates come from relative helpers. About 65 tests added across 15 files, and the full suite stays green.

1 files changed, 42 insertions, 0 deletions

diff --git a/tests/test-calendar-sync--apply-single-exception.el b/tests/test-calendar-sync--apply-single-exception.el
index 2fcf7c718..3d2342708 100644
--- a/tests/test-calendar-sync--apply-single-exception.el
+++ b/tests/test-calendar-sync--apply-single-exception.el
@@ -63,5 +63,47 @@
     (let ((result (calendar-sync--apply-single-exception occ exc)))
       (should (equal "Keep" (plist-get result :summary))))))
 
+;;; Normal Cases — remaining overridable fields
+
+(ert-deftest test-calendar-sync--apply-single-exception-overrides-description ()
+  "Normal: an exception :description overrides the occurrence's."
+  (let ((occ (list :start '(2026 3 15 14 0) :description "old"))
+        (exc (list :start '(2026 3 15 14 0) :description "new")))
+    (should (equal "new"
+                   (plist-get (calendar-sync--apply-single-exception occ exc)
+                              :description)))))
+
+(ert-deftest test-calendar-sync--apply-single-exception-overrides-location ()
+  "Normal: an exception :location overrides the occurrence's."
+  (let ((occ (list :start '(2026 3 15 14 0) :location "Room A"))
+        (exc (list :start '(2026 3 15 14 0) :location "Room B")))
+    (should (equal "Room B"
+                   (plist-get (calendar-sync--apply-single-exception occ exc)
+                              :location)))))
+
+(ert-deftest test-calendar-sync--apply-single-exception-overrides-attendees ()
+  "Normal: an exception :attendees overrides the occurrence's."
+  (let ((occ (list :start '(2026 3 15 14 0) :attendees '("a")))
+        (exc (list :start '(2026 3 15 14 0) :attendees '("b" "c"))))
+    (should (equal '("b" "c")
+                   (plist-get (calendar-sync--apply-single-exception occ exc)
+                              :attendees)))))
+
+(ert-deftest test-calendar-sync--apply-single-exception-overrides-organizer ()
+  "Normal: an exception :organizer overrides the occurrence's."
+  (let ((occ (list :start '(2026 3 15 14 0) :organizer "old@x"))
+        (exc (list :start '(2026 3 15 14 0) :organizer "new@x")))
+    (should (equal "new@x"
+                   (plist-get (calendar-sync--apply-single-exception occ exc)
+                              :organizer)))))
+
+(ert-deftest test-calendar-sync--apply-single-exception-overrides-url ()
+  "Normal: an exception :url overrides the occurrence's."
+  (let ((occ (list :start '(2026 3 15 14 0) :url "http://old"))
+        (exc (list :start '(2026 3 15 14 0) :url "http://new")))
+    (should (equal "http://new"
+                   (plist-get (calendar-sync--apply-single-exception occ exc)
+                              :url)))))
+
 (provide 'test-calendar-sync--apply-single-exception)
 ;;; test-calendar-sync--apply-single-exception.el ends here