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, 14 insertions, 0 deletions

diff --git a/tests/test-custom-line-paragraph-duplicate-line-or-region.el b/tests/test-custom-line-paragraph-duplicate-line-or-region.el
index bd82e00f..84f5bc2d 100644
--- a/tests/test-custom-line-paragraph-duplicate-line-or-region.el
+++ b/tests/test-custom-line-paragraph-duplicate-line-or-region.el
@@ -447,5 +447,19 @@
         (should (string-match-p "line\u000Cwith\u000Dcontrol\nline\u000Cwith\u000Dcontrol" (buffer-string))))
     (test-duplicate-line-or-region-teardown)))
 
+;;; Error Cases
+
+(ert-deftest test-duplicate-line-or-region-comment-without-syntax-errors ()
+  "Error: requesting a comment in a mode with no comment syntax signals
+user-error rather than producing malformed output."
+  (test-duplicate-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (fundamental-mode)  ; no comment-start defined
+        (insert "line one")
+        (goto-char (point-min))
+        (should-error (cj/duplicate-line-or-region t) :type 'user-error))
+    (test-duplicate-line-or-region-teardown)))
+
 (provide 'test-custom-line-paragraph-duplicate-line-or-region)
 ;;; test-custom-line-paragraph-duplicate-line-or-region.el ends here