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.

3 files changed, 162 insertions, 0 deletions

diff --git a/tests/test-prog-c-compile-command.el b/tests/test-prog-c-compile-command.el
new file mode 100644
index 00000000..59b5f3b6
--- /dev/null
+++ b/tests/test-prog-c-compile-command.el
@@ -0,0 +1,85 @@
+;;; test-prog-c-compile-command.el --- Tests for C compile command setup -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Verifies single-file C compile commands use file paths rather than buffer
+;; names, and quote paths safely for compile.el's shell command.
+
+;;; Code:
+
+(require 'ert)
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'prog-c)
+
+(ert-deftest test-prog-c-single-file-command-normal-path ()
+  "A simple C file should keep the readable historical command shape."
+  (should (equal
+           (cj/c--single-file-compile-command "/tmp/project/main.c")
+           "gcc -Wall -Wextra -g -o /tmp/project/main /tmp/project/main.c")))
+
+(ert-deftest test-prog-c-single-file-command-quotes-spaces ()
+  "C source and output paths with spaces should be shell-escaped."
+  (should (equal
+           (cj/c--single-file-compile-command "/tmp/my project/hello world.c")
+           "gcc -Wall -Wextra -g -o /tmp/my\\ project/hello\\ world /tmp/my\\ project/hello\\ world.c")))
+
+(ert-deftest test-prog-c-single-file-command-quotes-metacharacters ()
+  "Shell metacharacters in C paths should be escaped."
+  (should (equal
+           (cj/c--single-file-compile-command "/tmp/project/weird;name.c")
+           "gcc -Wall -Wextra -g -o /tmp/project/weird\\;name /tmp/project/weird\\;name.c")))
+
+(ert-deftest test-prog-c-single-file-command-errors-without-file ()
+  "Non-file buffers should fail clearly instead of compiling buffer names."
+  (should-error (cj/c--single-file-compile-command nil)
+                :type 'user-error))
+
+(ert-deftest test-prog-c-compile-command-uses-buffer-file-name ()
+  "The fallback compile command should ignore a renamed buffer's display name."
+  (let ((source (expand-file-name "real source.c" temporary-file-directory)))
+    (with-temp-buffer
+      (setq buffer-file-name source)
+      (rename-buffer "renamed display buffer" t)
+      (cl-letf (((symbol-function 'locate-dominating-file)
+                 (lambda (&rest _args) nil)))
+        (cj/c-compile-command))
+      (should (equal
+               compile-command
+               (format "gcc -Wall -Wextra -g -o %s %s"
+                       (shell-quote-argument
+                        (file-name-sans-extension source))
+                       (shell-quote-argument source)))))))
+
+(ert-deftest test-prog-c-compile-command-makefile-uses-make ()
+  "Normal: a Makefile dominating the source dir produces a make compile command."
+  (with-temp-buffer
+    (cl-letf (((symbol-function 'locate-dominating-file)
+               (lambda (_dir name)
+                 (when (equal name "Makefile") "/tmp/projectroot/"))))
+      (cj/c-compile-command))
+    (should (equal compile-command
+                   (format "cd %s && make -k "
+                           (shell-quote-argument "/tmp/projectroot/"))))))
+
+(ert-deftest test-prog-c-compile-command-cmake-uses-cmake-build ()
+  "Normal: a CMakeLists.txt dominating the source dir produces a cmake build command."
+  (with-temp-buffer
+    (cl-letf (((symbol-function 'locate-dominating-file)
+               (lambda (_dir name)
+                 (when (equal name "CMakeLists.txt") "/tmp/cmakeroot/"))))
+      (cj/c-compile-command))
+    (should (equal compile-command
+                   (format "cd %s && cmake --build build "
+                           (shell-quote-argument "/tmp/cmakeroot/"))))))
+
+(ert-deftest test-prog-c-compile-command-makefile-path-with-spaces-is-quoted ()
+  "Boundary: a Makefile in a directory with spaces is shell-quoted in the cd target."
+  (with-temp-buffer
+    (cl-letf (((symbol-function 'locate-dominating-file)
+               (lambda (_dir name)
+                 (when (equal name "Makefile") "/tmp/my project/"))))
+      (cj/c-compile-command))
+    (should (equal compile-command
+                   "cd /tmp/my\\ project/ && make -k "))))
+
+(provide 'test-prog-c-compile-command)
+;;; test-prog-c-compile-command.el ends here
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
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