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, 17 insertions, 0 deletions

diff --git a/tests/test-calendar-sync.el b/tests/test-calendar-sync.el
index b912c1328..62b00aba1 100644
--- a/tests/test-calendar-sync.el
+++ b/tests/test-calendar-sync.el
@@ -693,5 +693,22 @@ Valid events should be parsed, invalid ones skipped."
       (should retrieved)
       (should (eq 'ok (plist-get retrieved :status))))))
 
+;;; Tests: calendar-sync--parse-ics — boundary inputs
+
+(ert-deftest test-calendar-sync--parse-ics-nil-content-returns-nil ()
+  "Boundary: nil ICS content is handled gracefully and returns nil."
+  (should (null (calendar-sync--parse-ics nil))))
+
+(ert-deftest test-calendar-sync--parse-ics-drops-out-of-range-event ()
+  "Boundary: a non-recurring event outside the date range is dropped."
+  (let* ((far (test-calendar-sync-make-vevent
+               "OutOfRangeEvent"
+               (test-calendar-sync-time-days-from-now 3650 10 0)
+               (test-calendar-sync-time-days-from-now 3650 11 0)))
+         (ics (test-calendar-sync-make-ics far))
+         (org-content (calendar-sync--parse-ics ics)))
+    (should-not (and org-content
+                     (string-match-p "OutOfRangeEvent" org-content)))))
+
 (provide 'test-calendar-sync)
 ;;; test-calendar-sync.el ends here