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.

0 files changed, 0 insertions, 0 deletions