refactor(custom-editing): five hygiene fixes from the module-by-module re-review

- Guard `cj/duplicate-line-or-region' when COMMENT is non-nil but the
  current mode has no `comment-start' (e.g. fundamental-mode).
  Previously the function silently produced malformed output via
  `comment-region'; now it signals a clear `user-error'.

- Factor the `find-file' advice install in external-open.el into
  `cj/external-open-install-advice'.  Same idempotent shape
  (remove-then-add) but the intent is named.

- Add `cj/--validate-decoration-char' in custom-comments.el and
  wire it into all six divider / border / box helpers.  Rejects
  multi-char strings, empty strings, and control characters like
  newline/tab that would corrupt subsequent `M-q' flows.  Updated
  the five nil-decoration ERT tests from `:type 'wrong-type-argument'
  (the old crash signal from `string-to-char' on nil) to
  `:type 'user-error', since the validator produces a clear
  message instead of a deep crash.

- Extract `cj/--require-spell-checker' in flyspell-and-abbrev.el.
  Both `cj/flyspell-toggle' and `cj/flyspell-then-abbrev' now call
  the shared helper; the checker list lives in
  `cj/--spell-checker-executables', so adding nuspell or any other
  checker is a one-line edit.

- Preserve trailing newlines in custom-ordering output.  Both
  `cj/--arrayify' and `cj/--unarrayify' now detect a trailing
  newline on the input region and re-append it to the result,
  matching the pattern custom-text-enclose.el already uses.

1 files changed, 8 insertions, 1 deletions

diff --git a/modules/custom-line-paragraph.el b/modules/custom-line-paragraph.el
index 27f24cfe..0eb1e2a5 100644
--- a/modules/custom-line-paragraph.el
+++ b/modules/custom-line-paragraph.el
@@ -54,8 +54,15 @@
 
 (defun cj/duplicate-line-or-region (&optional comment)
   "Duplicate the current line or active region below.
-Comment the duplicated text when optional COMMENT is non-nil."
+Comment the duplicated text when optional COMMENT is non-nil.
+Signal `user-error' when COMMENT is non-nil but the current mode has
+no `comment-start' (e.g. `fundamental-mode'), since commenting would
+produce malformed output silently."
   (interactive "P")
+  (when (and comment (not (and (stringp comment-start)
+                                (> (length comment-start) 0))))
+    (user-error
+     "Cannot comment in %s: no comment syntax defined" major-mode))
   (let* ((b (if (region-active-p) (region-beginning) (line-beginning-position)))
          (e (if (region-active-p) (region-end) (line-end-position)))
          (lines (split-string (buffer-substring-no-properties b e) "\n")))