Fix deferred RPC span-tail replay ordering

[noise64]

• fixes p3 future-RPC replay when a completion marker resolves the RPC End before interleaved positional work reaches the replay cursor
• waits for the interleaved owners to advance normally, then atomically validates and consumes the RPC's adjacent FinishSpan by span ID
• applies the in-memory span transition only after the durable tail is consumed, without adding replay tolerance or skipping unrelated entries
• adds focused regressions for the observed timer/RPC ordering and for rejecting a mismatched span tail without advancing the cursor
• documents the post-terminal positional-tail ownership invariant in the durable-execution guide

Verification

• cargo fmt -p golem-worker-executor -- --check
• cargo test -p golem-worker-executor --lib prefetched_completion_ -- --report-time
• existing completion-prefetch and delivery-marker cursor-boundary tests

[netlify]

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[vigoo]

A redesign of how spans are recorded in the oplog is coming, solving this better