Fix macOS Apple Terminal split/resume spawn

The Apple Terminal branch used `open -a Terminal <binary> --args ...`, which
tells Terminal to open the binary as a document rather than executing it with
arguments. On a default macOS install (where Apple Terminal is the only
available terminal), split/resume would report success but never actually
launch jcode.

- Spawn Apple Terminal via osascript `do script` so jcode actually runs, cd-ing
  into the working dir and exec-ing the command (injecting JCODE_FRESH_SPAWN
  when needed, which the spawned shell would otherwise not inherit).
- Define an explicit macOS terminal preference list (ghostty, kitty, wezterm,
  alacritty, iterm2, then Apple Terminal last as the guaranteed fallback) and
  only try the modern terminals when actually installed/current.
- Add unit tests for the AppleScript/script generation.