Adaptive backpressure for hot-path GenServer.cast.

Probes the target mailbox depth via Process.info(pid, :message_queue_len) and switches behaviour when the queue crosses a watermark. See docs/adr/0013-event-dispatch-backpressure.md for the design rationale.

Policies

• :call_when_full -- escalate to a synchronous GenServer.call/3, applying backpressure to the caller. Use when message loss is unacceptable (input dispatch, test determinism).
• :drop_when_full -- drop the message and return {:dropped, :overflow}. Use when the consumer naturally recovers from drops (frame batching, idempotent updates).
• :fail_when_full -- same return shape as :drop_when_full; the policy field in telemetry metadata distinguishes "expected drop" from "explicit failure" for handler-side severity routing.

Telemetry

Emits [:raxol, :runtime, :backpressure] on every invocation.

• Measurements: %{queue_len: non_neg_integer()}
• Metadata: %{label, policy, decision, target} where decision is one of :cast, :call, :drop, :no_proc.

Examples

Backpressure.cast(MyDispatcher, {:dispatch, event},
  label: :dispatcher_input,
  policy: :call_when_full,
  watermark: 1_000
)