overBudgetAddressedSendIsRefusedNotLeaked

A payload one byte over the budget is refused by the seam with us.tractat.kuilt.core.PayloadTooLarge, not leaked as the fabric's own frame error.

The distinction is the whole point of publishing a budget: PayloadTooLarge names the number the caller should have respected, whereas a fabric-level error names a limit the caller had no way to read — and, in the two in-tree fabric seams before #2069, arrived only after the send had reported success, having torn the seam down or evicted a healthy peer on the way.

Gated on SeamCapabilities.supportsSendTo: broadcast is best-effort and drops an over-budget payload by contract, so only the addressed send has a refusal to observe.