refactor: rework gloss-add UX to single side-window buffer

The previous shape opened a regular (non-side) buffer for body input
and showed the saved entry in the side window after C-c C-c. That
left an extra window split during the typing phase and a side popup
the user didn't ask for.

New shape, modeled on `org-capture':

- `gloss-add' renders the term and underline as a read-only header in
  *gloss-add: TERM*, leaves the body region beneath it editable, and
  pops the buffer in the side-window slot. Point lands at the body
  start so the user can type immediately.
- `gloss-add-finish' reads the body via the `gloss-add--body-start'
  marker, saves with source `manual', kills the buffer, closes the
  side window, and echoes `gloss-add: saved TERM' for confirmation.
- `gloss-add-abort' kills the buffer and closes the side window.
- The shared `gloss--add-cleanup' helper handles kill + window-close
  for both finish and abort.

Read-only header uses text properties (`read-only', `front-sticky',
`rear-nonsticky') rather than narrowing, so the user can't escape the
restriction with `C-x n w'.

`gloss--add-finish-internal' no longer calls show-entry — the save
is its only responsibility. The display decision (show or not) is
the caller's, which lets `gloss-add-finish' choose "save and close"
while `gloss-lookup' still chooses "save and show."

The previous saved-window-config approach is dropped — the side-slot
takeover means there's nothing to restore. Layout returns to its
pre-add state on either C-c C-c or C-c C-k.

Adds `tests/test-gloss--add-flow-smoke.el' covering the four
interactive moments (open, finish, abort, empty-term guard) plus the
read-only-header invariant. Updates the
`gloss--add-finish-internal' tests to drop the show-entry assertion.

129 tests pass in 0.27s.

1 files changed, 19 insertions, 28 deletions

diff --git a/tests/test-gloss--add-finish-internal.el b/tests/test-gloss--add-finish-internal.el
index 688d6f2..bb7e4ee 100644
--- a/tests/test-gloss--add-finish-internal.el
+++ b/tests/test-gloss--add-finish-internal.el
@@ -15,47 +15,38 @@
 (require 'gloss)
 (require 'testutil-gloss)
 
-(ert-deftest test-gloss-add-finish-internal-saves-and-shows ()
-  "Normal: a fresh term + body is saved with source `manual' and shown."
+(ert-deftest test-gloss-add-finish-internal-saves-with-manual-source ()
+  "Normal: a fresh term + body is saved with source `manual'.
+Display is the caller's responsibility, not this function's."
   (gloss-test--with-missing-glossary
-    (let (shown)
-      (cl-letf (((symbol-function 'gloss-display-show-entry)
-                 (lambda (term body) (setq shown (list term body)))))
-        (gloss--add-finish-internal "newterm" "A new definition.")
-        (let ((saved (gloss-core-lookup "newterm")))
-          (should saved)
-          (should (equal (plist-get saved :body) "A new definition."))
-          (should (eq (plist-get saved :source) 'manual)))
-        (should (equal shown '("newterm" "A new definition.")))))))
+    (let ((saved (gloss--add-finish-internal "newterm" "A new definition.")))
+      (should saved)
+      (should (equal (plist-get saved :body) "A new definition."))
+      (should (eq (plist-get saved :source) 'manual))
+      (should (equal (gloss-core-lookup "newterm") saved)))))
 
 (ert-deftest test-gloss-add-finish-internal-empty-term-raises ()
   "Error: empty TERM raises `user-error'."
   (gloss-test--with-missing-glossary
-    (cl-letf (((symbol-function 'gloss-display-show-entry)
-               (lambda (_ _) nil)))
-      (should-error (gloss--add-finish-internal "" "Body.")
-                    :type 'user-error)
-      (should-error (gloss--add-finish-internal "   " "Body.")
-                    :type 'user-error))))
+    (should-error (gloss--add-finish-internal "" "Body.")
+                  :type 'user-error)
+    (should-error (gloss--add-finish-internal "   " "Body.")
+                  :type 'user-error)))
 
 (ert-deftest test-gloss-add-finish-internal-empty-body-raises ()
   "Error: empty BODY raises `user-error'."
   (gloss-test--with-missing-glossary
-    (cl-letf (((symbol-function 'gloss-display-show-entry)
-               (lambda (_ _) nil)))
-      (should-error (gloss--add-finish-internal "term" "")
-                    :type 'user-error)
-      (should-error (gloss--add-finish-internal "term" "   \n  ")
-                    :type 'user-error))))
+    (should-error (gloss--add-finish-internal "term" "")
+                  :type 'user-error)
+    (should-error (gloss--add-finish-internal "term" "   \n  ")
+                  :type 'user-error)))
 
 (ert-deftest test-gloss-add-finish-internal-trims-body-whitespace ()
   "Boundary: leading/trailing whitespace in BODY is trimmed before save."
   (gloss-test--with-missing-glossary
-    (cl-letf (((symbol-function 'gloss-display-show-entry)
-               (lambda (_ _) nil)))
-      (gloss--add-finish-internal "term" "  Body content.\n\n")
-      (let ((saved (gloss-core-lookup "term")))
-        (should (equal (plist-get saved :body) "Body content."))))))
+    (gloss--add-finish-internal "term" "  Body content.\n\n")
+    (let ((saved (gloss-core-lookup "term")))
+      (should (equal (plist-get saved :body) "Body content.")))))
 
 (provide 'test-gloss--add-finish-internal)
 ;;; test-gloss--add-finish-internal.el ends here