Problem: credentials were only set implicitly on first :CP submit.
There was no way to update wrong credentials, log out, or set
credentials ahead of time without editing the cache JSON manually.
Solution: add :CP login [platform] which always prompts for username
and password and overwrites any saved credentials for that platform.
Omitting the platform falls back to the active platform. Wire the
command through constants, parse_command, handle_command, and add
tab-completion (suggests platform names). Document in vimdoc under
the SUBMIT section and in the commands reference.
Add command parsing and dispatch for :CP race, :CP race stop, :CP stress,
and :CP submit. Add tab-completion for race (platform/contest/--lang),
stress (cwd executables at arg 2 and 3), and race stop. Add
<Plug>(cp-stress), <Plug>(cp-submit), and <Plug>(cp-race-stop) keymaps.
Problem: users who want keybindings must call vim.cmd('CP run') or
reach into internal Lua modules directly. There is no stable,
discoverable, lazy-load-friendly public API for key binding.
Solution: define 7 <Plug> mappings in plugin/cp.lua that dispatch
through the same handle_command() code path as :CP. Document them
in a new MAPPINGS section in the vimdoc with helptags and an example
config block.
