test(zigux): add lane18 install self-test runner

[adybag14-cyber]

Summary

• add zigux/tests/phase2_install_zig_selftest_contract_build.zig as a focused standalone Lane 18 build shard for PR test(zigux): add lane18 install self-test contract route #15310
• expose both a named phase2-install-zig-selftest-contract step and a plain test alias
• set the test run cwd to the repo root so the contract reads scripts/zigux/install-zig.py through its existing path

Why This Lane

• Lane 18 owns toolchain and action-path cleanup
• PR test(zigux): add lane18 install self-test contract route #15310 adds the Zig-facing install self-test contract, but it did not yet have the direct zig build route pattern used by nearby hardening follow-ups
• this runner keeps the change .zig-only and avoids reopening installer Python, workflows, policy JSON, docs, Makefiles, fixtures, or archive files

Validation

• authenticated PR reread confirmed base PR test(zigux): add lane18 install self-test contract route #15310 head deb3fa88779c21b7e8d5f170d6fb48cfd3f7f7ef
• authenticated branch reread confirmed zigux/tests/phase2_install_zig_selftest_contract_build.zig returned 404 on the base branch before publication
• focused scratch validation used the attached Zig 0.17.0-dev.87+9b177a7d2 toolchain with a current-like install self-test marker replay
• zig fmt --check phase2_install_zig_selftest_contract.zig phase2_install_zig_selftest_contract_build.zig
• zig build --build-file phase2_install_zig_selftest_contract_build.zig --list-steps
  • result: step list includes phase2-install-zig-selftest-contract and test
• zig build phase2-install-zig-selftest-contract --build-file phase2_install_zig_selftest_contract_build.zig --summary all
  • result: Build Summary: 3/3 steps succeeded; 2/2 tests passed
• zig build test --build-file phase2_install_zig_selftest_contract_build.zig --summary all
  • result: Build Summary: 3/3 steps succeeded; 2/2 tests passed
• authenticated compare against base PR branch:
  • changed file: zigux/tests/phase2_install_zig_selftest_contract_build.zig
  • additions: 33
  • deletions: 0
  • result: ahead_by: 1
  • result: behind_by: 0
  • result: status: ahead

Scope

• lane: Lane 18 -- Toolchain and action-path cleanup
• session issue: Zigux session 2026-05-29 16:34 UTC #15363
• base PR: test(zigux): add lane18 install self-test contract route #15310
• base branch: zigux-builder/lane18-install-selftest-contract-20260529t1534z-r3
• branch: zigux-builder/lane18-install-selftest-runner-20260529t1634z
• file:
  • zigux/tests/phase2_install_zig_selftest_contract_build.zig

Coordination

• keep PR test(zigux): add lane18 install self-test contract route #15310 as the separate install self-test contract body frontier
• keep PR test(zigux): add lane18 install archive-target contract #15255 as the separate install archive-target contract frontier
• keep PR Zigux session 2026-05-29 13:45 UTC - Lane 20 confdata EOF NUL proof #15199 as the separate install output contract frontier
• keep PR test(zigux): add lane18 kbuild routes contract #14811 as the separate kbuild routes contract frontier
• keep PR test(zigux): add lane18 cross selftest alignment contract #14755 as the separate cross selftest alignment contract frontier

Validation Boundary

Direct live checkout/raw GitHub fetch remains unavailable in this scheduled runtime, so executable proof used authenticated GitHub reads plus focused scratch validation matching the active contract markers.

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch zigux-builder/lane18-install-selftest-runner-20260529t1634z

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