fix(sequencer): only warn about missing proposed checkpoint once overdue

[spalladino]

Motivation

The orphan-block guard in checkSync (added in #23606) was logging at warn on every non-proposer validator, ~once per second for a full slot, every slot. Under pipelining a node receives and re-executes a block proposal for the next checkpoint up to one slot before the matching checkpoint proposal arrives, so the world-state tip legitimately sits in an as-yet-unproposed checkpoint for that whole window. That is the happy path, not the abnormal "proposer published blocks but never the checkpoint" case the guard is meant to flag. Observed on next-net: 118 warnings in ~59s on a healthy validator for a single slot.

Approach

The condition that distinguishes "checkpoint hasn't arrived yet" from "checkpoint will never arrive" is purely temporal — which is exactly what the archiver already computes in pruneOrphanProposedBlocks to decide when to prune an orphan block. The guard now reuses that same deadline: it still refuses to build (return undefined) whenever the orphan-shaped state holds, but only escalates to warn once the enclosing checkpoint is overdue by that deadline; within the normal pipelining window it logs at debug. The warn therefore fires at the same instant the archiver would prune the orphan.

Changes

• sequencer-client: Add isProposedCheckpointOverdue, mirroring the archiver's orphan-prune deadline (start of slot after the block's build slot + grace, grace derived from blockDurationMs as the node wiring does). Gate the existing guard's log level on it — warn when overdue, debug otherwise. Control flow is unchanged.
• sequencer-client (tests): Thread a real blockSlot through the orphan-guard test setup and split the warning test into an overdue case (expects warn) and a within-window case (expects no warn).