(new): expand runtime APIs

(fix): harden concurrent lifecycle
(tests): add regression coverage
(doc): update v1.2 guidance

runners.go

@@ -93,7 +93,7 @@ func (r Runner[T]) Every(timeout time.Duration) Runner[T] {
 // ExecRunners executes all runners registered on the Bot with context-based lifecycle management.
 //
 // Execution semantics by configuration:
-//   - every=0, async=true:  Runs once in a goroutine (fire and forget).
+//   - every=0, async=true:  Runs once in a goroutine; runtime shutdown waits for it.
 //   - every=0, async=false: Runs once synchronously; warns if slower than 2 seconds.
 //   - every>0, async=true:  Runs in a loop with the configured interval until ctx.Done().
 //   - every>0, async=false: Skipped with a warning (invalid configuration).
@@ -111,7 +111,7 @@ func (bot *Bot[T]) ExecRunners(ctx context.Context) {
 		}
 
 		if runner.every == 0 && runner.async {
-			// One-time async: fire and forget
+			// One-time async: non-blocking startup; runtime shutdown waits for completion.
 			bot.runnerOnceWG.Add(1)
 			go func(r Runner[T]) {
 				defer bot.runnerOnceWG.Done()