feat(examples): four runnable plugin examples

- player-load: load-on-join (PlayerReadyEvent → IO → modify).
- async-moderation: IAsyncEvent → coroutine bridge for chat moderation.
- periodic-leaderboard: pluginScope loop with parallel reads.
- bounty-board: kitchen-sink demo exercising every v0.1 primitive.

examples/periodic-leaderboard/README.md

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+# periodic-leaderboard
+
+Recomputes a top-5 leaderboard every minute by reading `PlayerStats.level`
+from every online player in parallel, sorting, and broadcasting.
+
+## What it shows
+
+- `pluginScope(this).launch { while (isActive) { delay(60.seconds); … } }` —
+  a plugin-lifetime periodic task. Cancels cleanly on shutdown via
+  `Async.shutdown()`.
+- Parallel `read` over players via `async { … }.awaitAll()`. Each `read`
+  switches to its player's owning world dispatcher independently — reads on
+  different worlds run concurrently, reads on the same world serialize.
+- `runCatching { … }` around the body so a single bad read doesn't kill the
+  loop.
+
+This is the shape for any plugin-wide background recompute: leaderboards,
+periodic backups of derived state, quest sync, etc.
+
+## Build
+
+```bash
+./gradlew shadowJar
+# → build/libs/periodic-leaderboard.jar
+```
+
+## Run
+
+1. Drop in `mods/`, wire `PlayerStatsBinding.componentType()` and
+   `onlinePlayers()`.
+2. For a faster demo while testing, change `delay(60.seconds)` to
+   `delay(5.seconds)` and rebuild.
+3. Connect 2+ players. The top-5 logs every loop tick.
+
+If you want **per-world** leaderboards instead, swap `pluginScope(this)` for
+`worldScope(world)` and run one loop per world. Cancellation flows through
+`WorldScopes.cancel(uuid)`.