Problem: `logger.log` positional args were hard to extend, and adding
`sync` support for pre-block notifications required a clean API. Test
stream completion had no user-visible signal. `setup_contest` could
silently overwrite files when a user's `filename` config returned
colliding paths.
Solution: Replace `(msg, level, override)` with `(msg, LogOpts?)` where
`LogOpts` carries `level`, `override`, and `sync`. Sync path calls
`vim.notify` directly; async path uses `vim.schedule` as before. Add
`on_done` callback to `scrape_all_tests`, fired via `on_exit` and
surfaced as a "Loaded N tests." notification. Detect filename collisions
in `proceed()` before touching the filesystem. Migrate all call sites.
Problem: closing the test editor left cp://test-N-* buffers alive,
causing E95 on reopen. The nofile buftype also rejected :w, which
was counterintuitive in an editable grid.
Solution: delete all test buffers in toggle_edit teardown. Switch
buftype to acwrite with a BufWriteCmd autocmd that persists test
cases and clears the modified flag. Hoist save_all_tests above
setup_keybindings so the autocmd closure can reference it.
