docs(audits): reorder-buffer bypass when delay-updates is off (#1886)

[oferchen]

Summary

• Audit confirming the ReorderBuffer bypass added for task Ensure release checksums are verifiable #1886 is correctly wired end-to-end through the threshold dispatcher.
• Traces both paths: ThresholdDeltaPipeline::new_bypass -> ParallelDeltaPipeline::new_bypass -> DeltaConsumer::spawn_bypass -> ReorderBuffer::passthrough (no spill), and the ordered path that retains the spill-aware design from task Ensure release workflow uploads architecture-specific assets #1884.
• Documents ServerConfig::write::delay_updates as the correct signal for the constructor choice at the dispatcher boundary.
• Flags the remaining integration gap (no production caller invokes ReceiverContext::set_delta_pipeline to upgrade the default SequentialDeltaPipeline) as a separate follow-up tracked outside this audit's scope.

Context

• Builds on PR feat(transfer): bypass ReorderBuffer when delay-updates is off (#1886) #3988 which introduced the passthrough / spawn_bypass / new_bypass constructors across engine and transfer layers.
• Companion to task Ensure release workflow uploads architecture-specific assets #1884's spill-to-tempfile design - this audit verifies the bypass path bypasses that spill, so high-concurrency transfers on systems with slow temp storage do not pay the spill cost when --delay-updates is off.
• Related adaptive-capacity work (PR feat: adapt parallel delta work queue to workload + core count #4012) introduces new_adaptive / new_bypass_adaptive constructors; the same flag-driven selection rule applies once those land.

Test plan

• CI: fmt + clippy + nextest pass (docs-only change; no code touched).
• Spot-check the cited file:line references against current master (the audit pins all citations to crates/engine/src/concurrent_delta/{reorder,consumer}.rs and crates/transfer/src/{delta_pipeline,reorder_buffer,receiver/mod,config/{mod,builder}}.rs).