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: with debug = true, there is not enough diagnostic output to
troubleshoot environment or execution issues. The resolved python path,
scraper commands, and compile/run shell commands are not logged.
Solution: add logger.log calls at key decision points: python env
resolution (nix vs uv vs discovery), uv sync stderr output, scraper
subprocess commands, and compile/run shell strings. All gated behind
the existing debug flag so they only appear when debug = true.
