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, 25 insertions, 0 deletions

diff --git a/tests/test-prog-c-mode-keybindings.el b/tests/test-prog-c-mode-keybindings.el
new file mode 100644
index 00000000..85b872b0
--- /dev/null
+++ b/tests/test-prog-c-mode-keybindings.el
@@ -0,0 +1,25 @@
+;;; test-prog-c-mode-keybindings.el --- Tests for cj/c-mode-keybindings -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Verifies the C mode hook installs the S-modifier overrides on the
+;; current buffer's local keymap.
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'prog-c)
+
+(ert-deftest test-prog-c-mode-keybindings-normal-installs-fkey-overrides ()
+  "Normal: cj/c-mode-keybindings binds S-F5 to cj/disabled and S-F6 to gdb."
+  (with-temp-buffer
+    (use-local-map (make-sparse-keymap))
+    (cj/c-mode-keybindings)
+    (should (eq (lookup-key (current-local-map) (kbd "S-<f5>"))
+                'cj/disabled))
+    (should (eq (lookup-key (current-local-map) (kbd "S-<f6>"))
+                'gdb))))
+
+(provide 'test-prog-c-mode-keybindings)
+;;; test-prog-c-mode-keybindings.el ends here