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/restore.lua

@@ -11,7 +11,7 @@ function M.restore_from_current_file()
   local current_file = (vim.uv.fs_realpath(vim.fn.expand('%:p')) or vim.fn.expand('%:p'))
   local file_state = cache.get_file_state(current_file)
   if not file_state then
-    logger.log('No cached state found for current file.', vim.log.levels.ERROR)
+    logger.log('No cached state found for current file.', { level = vim.log.levels.ERROR })
     return false
   end