Shared audit forwarder dispatcher.

Provides :auto/:async/:sync dispatch routing for audit forwarders (Phase 131, v1.29 SUITE-INTEGRATION — D-10, D-11). The routing semantics mirror Sigra.Delivery exactly; the per-forwarder :dispatch option key (NOT top-level :delivery_mode) is read from each forwarder's opts (D-07).

Dispatch Modes

• :auto — detects Oban presence via oban_running?/1; routes to :async when supervised, :sync when not.
• :async — enqueues a Sigra.Workers.AuditForward Oban job. Returns {:error, :async_worker_not_compiled} if the worker module is not compiled (Oban not in deps) — callers observe the degradation rather than receiving a silent :ok.
• :sync — calls forwarder_module.handle_event/4 inline in the calling process.

Custom Forwarders

Host-defined custom forwarders that always run :sync can call this dispatcher directly from their own handle_event/4 implementation:

def handle_event(_event, _measurements, metadata, opts) do
  Sigra.Audit.Forwarders.dispatch(__MODULE__, metadata, opts)
end

Placement (D-11)

The dispatcher is a standalone module (not a private fn on the worker) so custom always-:sync forwarders can reuse it without depending on the Oban worker being compiled. See RESEARCH.md §7.1 for the rationale.

Boot-Time Safety

The boot-time :async-without-Oban raise (D-26) does NOT live here — it lives in Sigra.Application.attach_forwarders/0. This dispatcher only routes already-validated :auto/:async/:sync values.