test(zigux): add lane16 rbtree bench replay runner

[adybag14-cyber]

Summary

• add zigux/tests/phase1_rbtree_bench_replay_build.zig as a focused standalone Lane 16 build shard for the existing plain phase1_rbtree_bench_replay.zig packet on PR #13383
• expose both a named phase1-rbtree-bench-replay step and a plain test alias without reopening zigux/tests/phase1_bench_build.zig
• keep the change one-file scoped and additive so it stays off the replay body, the shared bench packet, and tools/lib/rbtree.zig

Why This Lane

• Lane 16 still owns the bounded Phase 1 hardening-gates slice around bench-sensitive packets
• draft PR #13383 already owns zigux/tests/phase1_rbtree_bench_replay.zig, but it intentionally stopped at the replay file itself
• adding the dedicated runner is the smallest honest same-lane follow-through because it makes the rbtree replay directly runnable through its own build route without colliding with the other active rbtree replay fronts

Validation

• focused scratch validation used the attached Zigux-master.zip scaffold plus the attached Zig 0.17.0-dev.87+9b177a7d2 toolchain because this scheduled runtime cannot materialize a trustworthy live checkout through direct clone
• exact branch reread fetched zigux/tests/phase1_rbtree_bench_replay.zig from branch zigux-builder/lane16-rbtree-bench-replay-current-master-20260528t0648z before creating this stacked runner
• exact branch reread confirmed zigux/tests/phase1_rbtree_bench_replay_build.zig returned 404 Not Found before publication
• zig fmt --check zigux/tests/phase1_rbtree_bench_replay.zig zigux/tests/phase1_rbtree_bench_replay_build.zig tools/lib/rbtree.zig
• zig build --build-file zigux/tests/phase1_rbtree_bench_replay_build.zig --list-steps
  • result: step list includes phase1-rbtree-bench-replay and test
• zig build phase1-rbtree-bench-replay --build-file zigux/tests/phase1_rbtree_bench_replay_build.zig --summary all
  • result: Build Summary: 3/3 steps succeeded; 2/2 tests passed
  • result: phase1-rbtree-bench-replay success

Scope

• lane: Lane 16 — Phase 1 hardening gates
• session issue: #14328
• base PR: #13383
• branch: zigux-builder/lane16-rbtree-bench-replay-build-followup-20260528t2236z
• file:
  • zigux/tests/phase1_rbtree_bench_replay_build.zig
• intentionally no edits to zigux/tests/phase1_rbtree_bench_replay.zig, zigux/tests/phase1_bench.zig, zigux/tests/phase1_bench_build.zig, or tools/lib/rbtree.zig

Coordination

• keep draft PR #14266 as the separate findNextBit(...) replay runner frontier
• keep draft PR #14147 as the separate baseline find_bit replay runner frontier
• keep draft PR #14090 as the separate findNextAndBit(...) replay runner frontier
• keep draft PR #14031 as the separate findNextZeroBit(...) replay runner frontier
• keep draft PR #13972 as the separate string replay runner frontier
• keep draft PR #13915 as the separate find_bit edge replay runner frontier
• keep draft PR #13615 as the separate findNextZeroBit(...) replay frontier
• keep draft PR #13551 as the separate findNextAndBit(...) replay frontier
• keep draft PR #13493 as the separate baseline find_bit replay frontier
• keep draft PR #13383 as the separate plain rbtree replay frontier
• keep draft PR #13310 as the separate bitmap-weight replay frontier
• keep draft PR #13250 as the separate bitmap-window replay frontier
• keep draft PR #13134 as the separate hweight replay frontier
• keep draft PR #13074 as the separate cached rbtree findAddCached(...) replay frontier
• keep draft PR #13027 as the separate list_sort bench replay frontier
• keep draft PR #12959 as the separate rbtree postorder replay frontier
• keep draft PR #12902 as the separate duplicate-mutation replay frontier
• keep draft PR #12850 as the separate rbtree findAdd(...) replay frontier
• keep draft PR #12794 as the separate duplicate-range replay frontier
• keep draft PR #12566 as the separate rbtree bench guard frontier
• keep draft PR #12459 as the separate reminder-packet-members checker frontier
• keep draft PR #11043 as the separate current-bench-packet guard frontier
• keep draft PR #2203 as the separate bench-reminder checker path
• keep draft PR #2022 as the separate closure-gates workflow path

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch zigux-builder/lane16-rbtree-bench-replay-build-followup-20260528t2236z

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