test: cover C mode hooks and project compile branches

I added 7 new tests across 3 files, filling coverage gaps in `prog-c.el`. Two functions were untested (`cj/c-mode-settings`, `cj/c-mode-keybindings`) and `cj/c-compile-command` only had its single-file fallback covered.

`cj/c-compile-command` now has the Makefile and CMake branches tested, plus a Boundary case for a Makefile path with spaces being shell-quoted in the `cd` target. I added these to the existing `test-prog-c-compile-command.el` since the helper and dispatcher already lived there.

`cj/c-mode-settings` gets three tests. One covers the buffer-local invariants (`indent-tabs-mode` nil, `c-basic-offset` 4, `tab-width` 4, `fill-column` 80, `comment-auto-fill-only-comments` t). The other two cover the LSP branch: `lsp-deferred` runs when the function is fbound and `executable-find` returns a clangd path, and skips when clangd is missing.

`cj/c-mode-keybindings` gets one test asserting S-F5 binds to `cj/disabled` and S-F6 binds to `gdb` in the buffer's local keymap. No realistic Boundary or Error cases for installing two static bindings, so the single Normal case carries it.

I stubbed `auto-fill-mode`, `electric-pair-mode`, `lsp-deferred`, `executable-find`, and `locate-dominating-file` at the boundaries via `cl-letf`. Buffer-local state was exercised real in `with-temp-buffer`.

12 prog-c tests pass together: 5 existing plus 7 new.

1 files changed, 52 insertions, 0 deletions

diff --git a/tests/test-prog-c-mode-settings.el b/tests/test-prog-c-mode-settings.el
new file mode 100644
index 00000000..eef2d910
--- /dev/null
+++ b/tests/test-prog-c-mode-settings.el
@@ -0,0 +1,52 @@
+;;; test-prog-c-mode-settings.el --- Tests for cj/c-mode-settings -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Verifies the C mode hook applies the documented buffer-local values and
+;; only calls `lsp-deferred' when both the function and clangd are available.
+
+;;; Code:
+
+(require 'cl-lib)
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'prog-c)
+
+(ert-deftest test-prog-c-mode-settings-normal-applies-buffer-locals ()
+  "Normal: cj/c-mode-settings applies the documented buffer-local values."
+  (with-temp-buffer
+    (cl-letf (((symbol-function 'auto-fill-mode) (lambda (&rest _) nil))
+              ((symbol-function 'electric-pair-mode) (lambda (&rest _) nil))
+              ((symbol-function 'lsp-deferred) (lambda (&rest _) nil))
+              ((symbol-function 'executable-find) (lambda (_) nil)))
+      (cj/c-mode-settings))
+    (should (eq indent-tabs-mode nil))
+    (should (= c-basic-offset 4))
+    (should (= tab-width 4))
+    (should (= fill-column 80))
+    (should comment-auto-fill-only-comments)))
+
+(ert-deftest test-prog-c-mode-settings-boundary-lsp-available-deferred ()
+  "Boundary: when lsp-deferred is fbound and clangd is on PATH, lsp-deferred is called."
+  (let ((lsp-calls 0))
+    (with-temp-buffer
+      (cl-letf (((symbol-function 'auto-fill-mode) (lambda (&rest _) nil))
+                ((symbol-function 'electric-pair-mode) (lambda (&rest _) nil))
+                ((symbol-function 'lsp-deferred) (lambda () (cl-incf lsp-calls)))
+                ((symbol-function 'executable-find) (lambda (_) "/usr/bin/clangd")))
+        (cj/c-mode-settings)))
+    (should (= lsp-calls 1))))
+
+(ert-deftest test-prog-c-mode-settings-error-clangd-missing-skips-lsp ()
+  "Error: when clangd is not on PATH, lsp-deferred is NOT called."
+  (let ((lsp-calls 0))
+    (with-temp-buffer
+      (cl-letf (((symbol-function 'auto-fill-mode) (lambda (&rest _) nil))
+                ((symbol-function 'electric-pair-mode) (lambda (&rest _) nil))
+                ((symbol-function 'lsp-deferred) (lambda () (cl-incf lsp-calls)))
+                ((symbol-function 'executable-find) (lambda (_) nil)))
+        (cj/c-mode-settings)))
+    (should (zerop lsp-calls))))
+
+(provide 'test-prog-c-mode-settings)
+;;; test-prog-c-mode-settings.el ends here