## Problem
`_submit_sync` was a 170-line nested closure with `_solve_turnstile` and
the browser-install block further nested inside it. Status events went
to
stderr, which `run_scraper()` silently discards, leaving the user with a
10–30s silent hang after credential entry. The NDJSON spawn path also
lacked stdin support, so submit had no streaming path at all.
## Solution
Extract `_TURNSTILE_JS`, `_solve_turnstile`, `_ensure_browser`, and
`_submit_headless` to module level in `atcoder.py`; status events
(`installing_browser`, `checking_login`, `logging_in`, `submitting`) now
print to stdout as NDJSON. Add stdin pipe support to the NDJSON spawn
path in `scraper.lua` and switch `M.submit` to streaming with an
`on_status` callback. Wire `on_status` in `submit.lua` to fire
`vim.notify` for each phase transition.
Problem: credentials were stored in a separate file,
cp-nvim-credentials.json, alongside the main cp-nvim.json cache.
Two files for one plugin's persistent state was unnecessary.
Solution: add get_credentials/set_credentials to cache.lua, storing
credentials under _credentials[platform] in the shared cache. Update
clear_all() to preserve _credentials across cache wipes. Remove the
separate file, load_credentials, and save_credentials from submit.lua.
Add submit.lua that reads credentials from a local JSON store (prompting
via vim.ui.input/inputsecret on first use), reads the source file, and
delegates to scraper.submit(). Add language_ids.py with platform-to-
language-ID mappings for atcoder, codeforces, and cses.
