Fan out rust-ci-full nextest by platform

[starr-openai]

Why

rust-ci-full was paying the full Cargo nextest build-and-run cost once per platform, with Windows ARM64 as the long pole. This change moves the heavy work into one reusable per-platform flow: build a nextest archive once, then replay it across four shards so the platform lane spends less time running tests serially. For Windows ARM64, the archive is cross-compiled on Windows x64 and replayed on native Windows ARM64 shards so the slow ARM64 machine is used for execution rather than compilation.

What changed

• split the rust-ci-full nextest matrix into five explicit per-platform reusable-workflow calls
• add .github/workflows/rust-ci-full-nextest-platform.yml to build one archive, upload timings/helpers, replay four nextest shards, upload per-shard JUnit, and roll the shard status back up per platform
• add Windows CI helpers for Dev Drive setup and MSVC ARM64 linker environment export so the Windows ARM64 archive can be produced on Windows x64
• keep the existing Cargo git CLI fetch hardening inside the reusable workflow, since caller workflow-level env does not flow through workflow_call
• document the archive-backed shard shape in .github/workflows/README.md
• raise the default nextest slow timeout to 30s so the sharded full-CI path does not treat every >15s test as stuck

Verification

• validated the archive/shard flow with live GitHub Actions runs on this PR branch
• Windows ARM64 cross-compile latency on completed runs:
  • https://github.com/openai/codex/actions/runs/26118759651: 34m30s lane e2e, 17m16s archive build, 9m55s shard phase
  • https://github.com/openai/codex/actions/runs/26120777976: 30m36s lane e2e, 17m21s archive build, 6m50s shard phase
• comparable pre-cross-compile sharded Windows ARM64 runs were 55m01s, 50m21s, and 46m42s, so the completed cross-compile runs improved the lane by roughly 12m to 24m versus the prior range
• latest corrected cross-compile run: https://github.com/openai/codex/actions/runs/26120777976
  • Windows ARM64 archive built successfully on Windows x64
  • native Windows ARM64 shards started immediately after the archive upload
  • 3/4 Windows ARM64 shards passed; the failing shard hit the same existing code_mode test failure seen outside this lane
• downloaded failed-shard JUnit XML from the validation runs and confirmed the remaining red is from known test failures, not archive/shard wiring
• no local Codex tests run per repo guidance

Notes

• this PR does not change developers.openai.com documentation