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, 37 insertions, 0 deletions

diff --git a/tests/test-coverage-core--project-root.el b/tests/test-coverage-core--project-root.el
new file mode 100644
index 000000000..9d596217a
--- /dev/null
+++ b/tests/test-coverage-core--project-root.el
@@ -0,0 +1,37 @@
+;;; test-coverage-core--project-root.el --- Tests for cj/--coverage-project-root -*- lexical-binding: t -*-
+
+;;; Commentary:
+;; Tests for `cj/--coverage-project-root' in coverage-core.el — returns the
+;; projectile project root when available, else `default-directory'.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'coverage-core)
+
+;;; Normal Cases
+
+(ert-deftest test-coverage-project-root-uses-projectile-when-available ()
+  "Normal: with projectile available and in a project, returns its root."
+  (cl-letf (((symbol-function 'projectile-project-root)
+             (lambda () "/home/u/proj/")))
+    (should (equal (cj/--coverage-project-root) "/home/u/proj/"))))
+
+;;; Boundary Cases
+
+(ert-deftest test-coverage-project-root-falls-back-when-projectile-absent ()
+  "Boundary: with no projectile function, falls back to default-directory."
+  (cl-letf (((symbol-function 'projectile-project-root) nil))
+    (let ((default-directory "/fallback/dir/"))
+      (should (equal (cj/--coverage-project-root) "/fallback/dir/")))))
+
+(ert-deftest test-coverage-project-root-falls-back-when-not-in-project ()
+  "Boundary: projectile present but returns nil (not in a project) falls back."
+  (cl-letf (((symbol-function 'projectile-project-root) (lambda () nil)))
+    (let ((default-directory "/fallback/dir/"))
+      (should (equal (cj/--coverage-project-root) "/fallback/dir/")))))
+
+(provide 'test-coverage-core--project-root)
+;;; test-coverage-core--project-root.el ends here