* set C standard to C11
* create kart/kart_cli symlinks in dev, not just bundle
* enable debug via `-DCMAKE_BUILD_TYPE=Debug`

Enable via -DCMAKE_BUILD_TYPE=Debug at build-time and setting
KART_HELPER_DEBUG=1 in the environment.

Search for the kart executable via OS-specific methods if possible,
falling back to argv[0]. Then look for sibling kart_cli. Then try again
after re-resolving symlinks.

If Kart is built with -DCLI_HELPER=ON then make it on by default.

* enable test-run helper logging and dump log file after e2e tests
* turn off/on explicitly to deal with helper mode being default enabled

* handle semget() flags correctly per-OS
* add pid to log messages
* make semnum a constant
* build with -Wall & -Werror

* write to helper log from helper & children, resolve to absolute path so that cwd changes don't break
* more detailed logging around forks/pids, and exit/semaphore messaging
* `union semun` is defined differently on Linux & macOS... even though they're effectively the same, reflect accurately

On macOS/arm64, it's critical to define the final argument to semctl()
as variadic via ctypes (ie: don't define it), because the calling
convention differs between fixed and variadic arguments.
python/cpython#92892

we set it to ignore in the helper itself, but if the execution
kart process invokes a subprocess (eg: git) and interacts via
asyncio, then the subprocess exit code is lost on Linux.

Reset it to the default after the fork happens.

Means local development builds don't have gotchas wrt code-reloading,
but CI & release builds include it. Add a note to the contributing docs.