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, 18 insertions, 0 deletions

diff --git a/tests/test-calendar-sync--get-all-property-lines.el b/tests/test-calendar-sync--get-all-property-lines.el
index c95041c9..737d2af0 100644
--- a/tests/test-calendar-sync--get-all-property-lines.el
+++ b/tests/test-calendar-sync--get-all-property-lines.el
@@ -57,5 +57,23 @@
   "Test empty event string returns nil."
   (should (null (calendar-sync--get-all-property-lines "" "ATTENDEE"))))
 
+;;; Boundary Cases — position advancement
+
+(ert-deftest test-calendar-sync--get-all-property-lines-property-at-end-no-newline ()
+  "Boundary: a match at end of string with no trailing newline still returns it.
+Exercises the end-equals-length branch of position advancement."
+  (let ((result (calendar-sync--get-all-property-lines
+                 "ATTENDEE:foo@example.com" "ATTENDEE")))
+    (should (= 1 (length result)))
+    (should (string-match-p "foo@example.com" (car result)))))
+
+(ert-deftest test-calendar-sync--get-all-property-lines-second-match-after-continuation ()
+  "Boundary: a first match with a continuation does not hide the second match."
+  (let ((result (calendar-sync--get-all-property-lines
+                 "ATTENDEE:a\n more\nATTENDEE:b\nSUMMARY:x" "ATTENDEE")))
+    (should (= 2 (length result)))
+    (should (string-match-p "more" (nth 0 result)))
+    (should (string-match-p "ATTENDEE:b" (nth 1 result)))))
+
 (provide 'test-calendar-sync--get-all-property-lines)
 ;;; test-calendar-sync--get-all-property-lines.el ends here