test(zigux): add lane16 find_next_and_bit bench runner

[adybag14-cyber]

Summary

• add zigux/tests/phase1_find_next_and_bit_bench_replay_build.zig as a focused standalone Lane 16 build shard for the live phase1_find_next_and_bit_bench_replay.zig packet on PR #13551
• expose both a named phase1-find-next-and-bit-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 merged bench packet, helper bodies, checker scripts, fixtures, docs, and adjacent replay fronts

Why This Lane

• Lane 16 still owns the bounded Phase 1 hardening-gates slice around bench-sensitive packets
• draft PR #13551 already owns zigux/tests/phase1_find_next_and_bit_bench_replay.zig, but it intentionally stopped at the replay file itself
• adding the dedicated runner as a stacked one-file follow-up is the smallest honest same-lane move because it makes that replay directly runnable on its own build route without reopening the replay file, the shared bench packet, or tools/lib/find_bit.zig

Validation

• exact PR-branch reread fetched zigux/tests/phase1_find_next_and_bit_bench_replay.zig from branch zigux-builder/lane16-find-next-and-bit-bench-replay-current-master-20260528t0933z before creating this stacked runner
• 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 does not have a trustworthy live checkout
• overlaid only the exact replay file from PR #13551 plus this new runner into the scratch tree
• zig fmt --check phase1_find_next_and_bit_bench_replay.zig phase1_find_next_and_bit_bench_replay_build.zig ../../tools/lib/find_bit.zig
• zig build --build-file phase1_find_next_and_bit_bench_replay_build.zig --list-steps
  • result: step list includes phase1-find-next-and-bit-bench-replay and test
• zig build phase1-find-next-and-bit-bench-replay --build-file phase1_find_next_and_bit_bench_replay_build.zig --summary all
  • result: Build Summary: 3/3 steps succeeded; 2/2 tests passed
• zig build test --build-file phase1_find_next_and_bit_bench_replay_build.zig --summary all
  • result: Build Summary: 3/3 steps succeeded; 2/2 tests passed

Scope

• lane: Lane 16 — Phase 1 hardening gates
• session issue: #15088
• base PR: #13551
• branch: zigux-builder/lane16-find-next-and-bit-bench-replay-build-followup-20260529t1137z
• file:
  • zigux/tests/phase1_find_next_and_bit_bench_replay_build.zig
• intentionally no edits to zigux/tests/phase1_find_next_and_bit_bench_replay.zig, zigux/tests/phase1_bench.zig, zigux/tests/phase1_bench_build.zig, tools/lib/find_bit.zig, checker scripts, fixtures, docs, or workflows

Coordination

• keep draft PR #14031 as the separate findNextZeroBit(...) replay runner frontier
• keep draft PR #13915 as the separate find_bit edge replay runner frontier
• keep draft PR #13493 as the separate baseline find_bit replay frontier
• keep draft PR #13383 as the separate baseline 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-find-next-and-bit-bench-replay-build-followup-20260529t1137z

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