Speed up Windows x64 Rust tests with archive fanout

[starr-openai]

Summary

• replace the monolithic Windows x64 Rust test row in rust-ci-full with one Windows x64 cargo nextest archive producer plus six archive-backed Windows x64 test shards
• keep the producer and shard jobs on the default codex-windows-x64 runner label
• wire the full-ci gatherer to the new Windows x64 shard matrix

Why

The old Windows x64 Rust full-ci path compiled and ran tests inside one long Windows job. This change builds the nextest archive once, uploads it as a short-lived artifact, then runs partitioned test shards on separate Windows hosts.

Evidence

• non-XL 6x8 probe run showed roughly 17m17s archive-builder-plus-shards wall time from producer start to last shard completion
• the latest pre-rebase PR run had unrelated Bazel failure; this workflow change is validated by rust-ci-full shape rather than local tests

Validation

• not run locally; GitHub Actions is the validation path for this workflow-only change