Phase 3 Plan 05 invoice write surface (BILL-17/18/19).

Exposes the five user-path invoice actions on Accrue.Billing via the Plan 01 defdelegate facade: finalize_invoice, void_invoice, pay_invoice, mark_uncollectible, send_invoice. Each follows the D3-18 one-shape Repo.transact/2 pattern:

telemetry.span
  |> Repo.transact (
       Processor.<op>(stripe_id, opts)
       |> InvoiceProjection.decompose/1
       |> update row via Invoice.changeset/2 (user path, enforces transitions)
       |> upsert child items by stripe_id
       |> Events.record/1 (same transaction — EVT-04)
     )
  |> IntentResult.wrap (pay_invoice only — may surface SCA)

pay_invoice/2 returns an intent_result tagged union because Stripe may surface SCA/3DS; the other four return plain {:ok, %Invoice{}}. Every action has a bang variant that raises on {:error, _}; pay_invoice!/2 additionally raises Accrue.ActionRequiredError on :requires_action.

The webhook path uses the force-status bypass on the Invoice schema — that bypass is NOT reachable from this module. Illegal user-path transitions (e.g. draft -> paid) are rejected by Invoice.changeset/2 with an error on :status and propagate as {:error, %Ecto.Changeset{}}.