Shard CI_iOS test suites

[bradleymackey]

Summary

This updates the iOS CI workflow so CI_iOS builds test products once, uploads them as a tarred artifact, then runs each test target as a parallel test-without-building shard.

Why

The previous workflow stopped at build-for-testing, so it compiled iOS test products but did not run the test suites. Running everything serially in one job would add significant wall-clock time. Sharding keeps full test coverage while reducing elapsed time to roughly build time plus the slowest suite and artifact transfer.

Impact

The required CI_iOS check name is preserved as an aggregate job. Individual shard jobs expose failures per test target, and the build artifact uses RUNNER_TEMP to avoid workspace churn.

Validation

• make format
• make lint
• ruby YAML parse for .github/workflows/validate-all.yml
• git diff --check
• local xcodebuild test-without-building enumeration for VaultCoreTests