refactor(logger): table-based LogOpts; add sync, on_done to test stream

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.

lua/cp/runner/execute.lua

@@ -52,7 +52,7 @@ function M.compile(compile_cmd, substitutions, on_complete)
     r.stdout = ansi.bytes_to_string(r.stdout or '')
 
     if r.code == 0 then
-      logger.log(('Compilation successful in %.1fms.'):format(dt), vim.log.levels.INFO)
+      logger.log(('Compilation successful in %.1fms.'):format(dt), { level = vim.log.levels.INFO })
     else
       logger.log(('Compilation failed in %.1fms.'):format(dt))
     end