test: close coverage gaps from the preceding batch

Untested paths surfaced while reviewing the preceding feature/fix commits:
- calendar-sync: a test that `-L' precedes `-l' in the worker command (separate `member' checks wouldn't catch a swap), plus a `:slow' tag on the real-subprocess worker test so it stays out of the default `make test' run.
- org-capture cache: a killed marker buffer invalidates the entry and the next resolution rescans without erroring on the stale marker, `cj/org-capture-clear-target-cache' actually empties the hash, and non-`file+headline' targets (`file', `file+olp', `file+function') fall through to the original `org-capture-set-target-location'.
- lorem-optimum: `cj/lipsum-title' on an empty chain returns "", not an error.
- calibredb-epub: a negative `cj/nov-margin-percent' is clamped up to 0 (text takes the full window width).
- mu4e attachments: the default save directory comes from the part's `:target-dir' and falls back to `~/Downloads/', and asking for the attachment at point on a header line fails with a `user-error'.

1 files changed, 6 insertions, 0 deletions

diff --git a/tests/test-lorem-optimum.el b/tests/test-lorem-optimum.el
index 40bdc684..f928c972 100644
--- a/tests/test-lorem-optimum.el
+++ b/tests/test-lorem-optimum.el
@@ -247,5 +247,11 @@
       (should (> (length result) 0))
       (should (string-match-p "^[[:upper:]]" result)))))
 
+(ert-deftest test-title-generation-empty-chain-returns-empty-string ()
+  "Boundary: an empty Markov chain (`cj/markov-random-key' returns nil) yields
+an empty string, not an error."
+  (let ((cj/lipsum-chain (cj/markov-chain-create)))
+    (should (equal "" (cj/lipsum-title)))))
+
 (provide 'test-lorem-optimum)
 ;;; test-lorem-optimum.el ends here