Define sentinel error types (ErrNotRunning, ErrRateLimit, ErrTemporary,
ErrSendFailed) so the Manager can classify Send failures and choose the
right retry strategy: permanent errors bail immediately, rate-limit
errors use a fixed 1s delay, and temporary/unknown errors use exponential
backoff (500ms→1s→2s, capped at 8s, up to 3 retries). A per-channel
token-bucket rate limiter (golang.org/x/time/rate) throttles outbound
sends before they hit the platform API.
Hoshina