fix: isolate explicit local registry cache

[rudijetson]

Closes #21

Summary

• resolve the current workspace @learnrudi/env package from registry-client, so child processes honor an explicit RUDI_HOME
• make a valid RUDI_REGISTRY_ROOT authoritative instead of allowing a newer cache from another root to win
• add an isolated cross-process regression that exits before any fetch when cache isolation is broken
• refresh the checked-in CLI bundle

Failure evidence

The same resolver fixture race appeared in two otherwise unrelated PRs:

• intermittent run that passed on rerun
• repeated failure on PR #18

The regression proved two sequential failures: package-local environment resolution pointed at the real follower cache, then a newer temporary cache overrode the explicit local root.

Verification

• red/green focused regression
• 30 Registry client tests
• 7 resolver-related tests
• 100 consecutive repetitions of the formerly flaky suite
• pnpm test: 634 passed
• reproducible pnpm build
• architecture debt scan: 0 findings
• npm pack --dry-run --json: 6 expected files
• git diff --check

No installed Registry state or primary dirty checkout was modified.