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, 73 insertions, 0 deletions

diff --git a/tests/test-show-kill-ring--insert-item.el b/tests/test-show-kill-ring--insert-item.el
new file mode 100644
index 000000000..a29ca75e6
--- /dev/null
+++ b/tests/test-show-kill-ring--insert-item.el
@@ -0,0 +1,73 @@
+;;; test-show-kill-ring--insert-item.el --- Tests for show-kill-insert-item -*- lexical-binding: t -*-
+
+;;; Commentary:
+;; Tests for `show-kill-insert-item' in show-kill-ring.el — inserts a
+;; kill-ring entry into the current buffer, truncating to
+;; `show-kill-max-item-size' with an ellipsis when too long.  The ellipsis
+;; sits inline for short items and on its own line for items wider than the
+;; frame.  Frame width is read at runtime so the test is environment-stable.
+
+;;; Code:
+
+(require 'ert)
+(require 'show-kill-ring)
+
+;;; Normal Cases
+
+(ert-deftest test-show-kill-ring-insert-item-short-verbatim ()
+  "Normal: an item shorter than the max is inserted unchanged."
+  (let ((show-kill-max-item-size 1000))
+    (with-temp-buffer
+      (show-kill-insert-item "hello")
+      (should (string= (buffer-string) "hello")))))
+
+(ert-deftest test-show-kill-ring-insert-item-inline-ellipsis ()
+  "Normal: an over-max item narrower than the frame gets an inline ellipsis."
+  (let* ((show-kill-max-item-size 5)
+         (len (/ (frame-width) 2))        ; > max, < (frame-width - 5)
+         (item (make-string len ?b)))
+    (with-temp-buffer
+      (show-kill-insert-item item)
+      (should (string= (buffer-string) "bbbbb...")))))
+
+;;; Boundary Cases
+
+(ert-deftest test-show-kill-ring-insert-item-length-equals-max-truncates ()
+  "Boundary: length exactly equal to max truncates — the guard is (< len max)."
+  (let ((show-kill-max-item-size 5))
+    (with-temp-buffer
+      (show-kill-insert-item "hello")     ; length 5, equals max
+      (should (string= (buffer-string) "hello...")))))
+
+(ert-deftest test-show-kill-ring-insert-item-wide-newline-ellipsis ()
+  "Boundary: an item wider than the frame puts the ellipsis on its own line."
+  (let* ((show-kill-max-item-size 5)
+         (item (make-string (+ (frame-width) 10) ?a)))
+    (with-temp-buffer
+      (show-kill-insert-item item)
+      (should (string= (buffer-string) "aaaaa\n...")))))
+
+(ert-deftest test-show-kill-ring-insert-item-max-nil-verbatim ()
+  "Boundary: a non-numeric max disables truncation."
+  (let ((show-kill-max-item-size nil))
+    (with-temp-buffer
+      (show-kill-insert-item "anything long enough to exceed nothing")
+      (should (string= (buffer-string)
+                       "anything long enough to exceed nothing")))))
+
+(ert-deftest test-show-kill-ring-insert-item-max-negative-verbatim ()
+  "Boundary: a negative max disables truncation."
+  (let ((show-kill-max-item-size -1))
+    (with-temp-buffer
+      (show-kill-insert-item "abc")
+      (should (string= (buffer-string) "abc")))))
+
+(ert-deftest test-show-kill-ring-insert-item-empty-string ()
+  "Boundary: an empty item inserts nothing and does not error."
+  (let ((show-kill-max-item-size 1000))
+    (with-temp-buffer
+      (show-kill-insert-item "")
+      (should (string= (buffer-string) "")))))
+
+(provide 'test-show-kill-ring--insert-item)
+;;; test-show-kill-ring--insert-item.el ends here