Produce workflow branch variables at their natural step via a provider seam

Implements the WorkflowVariableProvider seam for #3613003.

• WorkflowVariableProvider tagged-service seam + WorkflowVariableProducer collector: a submodule contributes process variables computed from the finalized order, produced at the prepare step rather than guessed at workflow start.
• SettlementVariableProvider produces yoyaku_settlement_manual (manual only when every active line settles manually).
• PrepareWorkflowVariables action invokes the producer at the prepare step; the cancellation logic moves from the removed PrepareCancellation action to CancellationVariableProvider.
• Example workflow: the attendance flow is gated on yoyaku_settlement_manual == 1, with a direct flow to the confirmed end otherwise.

Still to do on this branch: a payment provider in the payment bridge (gate the deposit fork on the deposit amount and drop the optional payment dependency from the starter), an end-to-end workflow test, documentation and French translations.