fix: keep FCU timestamps at or ahead of wall clock

[meyer9]

Summary

• Replaces the 2-second threshold catchup with max(now, lastTimestamp) so FCU timestamps never fall behind wall clock
• Fixes basic-benchmarks CI flake where the builder sees "FCU arrived too late" on every block, produces empty blocks (0 flashblocks), and the transferonly worker times out waiting for a receipt

Root Cause

Each block cycle (FCU → build → getPayload → newPayload) takes slightly >1s of real time on slow CI runners. The timestamp increments by exactly 1s per block (lastTimestamp + 1), so it gradually drifts behind wall clock. The builder's calculate_flashblocks subtracts a leeway from the target timestamp, sees no remaining time, and builds 0 flashblocks — producing blocks with only the deposit tx and no pool transactions.

The old 2-second guard never triggered because the per-block drift is <2s, but it accumulates over many blocks.

[cb-heimdall]

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