A structured terminal failure, delivered to a Bloccs.call/4 / cast/4 caller as the {:error, t()} result when a request fails before it can reply.

Without this, a failed request would surface only as a call/4 timeout — the caller could not tell "failed" from "slow". The struct names where on the request's path the failure occurred:

• :validate — the payload failed a node's inbound Name@N schema.
• :execute — the node's pure_core / effect_shell raised, returned {:error, _}, or exceeded its timeout_ms.
• :dispatch — the node produced a result but delivering it downstream failed.

reason is the underlying failure term (an exception struct, a schema-error tuple, :timeout, a {:dispatch_failed, _} tuple, …). attempt is the retry attempt the failure occurred on (0 for the first).

Only terminal failures of a node on the request's 1:1 trace are reported — a failure that is retried is not (a later attempt may still succeed and reply). Correlation does not survive a [batch]/[join] (which mint a fresh trace_id); see Bloccs.Collector.