docs(design): reorder buffer metrics and --delay-updates bypass

[oferchen]

Summary

• Adds docs/design/reorderbuffer-metrics-and-bypass.md (499 lines) covering tasks Ensure release task uploads cross-compiled binaries #1885 (queue-depth and stall-duration metrics for BoundedReorderBuffer) and Ensure release checksums are verifiable #1886 (skip the buffer when --delay-updates is off).
• Part A specifies five u64 counters (current/peak depth, total/episode stall, derived mean), the two insert/drain_consecutive instrumentation hooks, single-threaded plain-u64 storage rationale, debug_log!(Pipeline, 2, ...) emission at -vv, and an opt-in --reorder-metrics-json flag.
• Part B specifies a BypassPipeline sibling to ParallelDeltaPipeline selected from WriteConfig::delay_updates, the correctness argument across NDX ack ordering, on-disk commit ordering, and the --delete-during directory fence (cross-references Ensure packages coexist with upstream rsync #1893), plus an identical-output fixture, unit test, and interop assertion.
• Recommends metrics-first ordering: bypass lands only after live peak_depth and stall_count data confirm the head-of-line stall is real.

Test plan

• markdownlint clean on the new doc
• Doc renders correctly on GitHub (tables, code blocks, internal links)
• No references to forbidden tooling terms or em-dashes