fix(mirror): handle osx-*-tar naming in Node.js index.json

[CalvinAllen]

Summary

• Node.js index.json uses osx-x64-tar and osx-arm64-tar for macOS entries (since v15+), but the mirror script only mapped darwin-x64 and darwin-arm64 — names that don't appear in the index
• This caused all darwin platform binaries to be skipped during mirroring, even though the actual tarballs exist on nodejs.org/dist/
• Added osx-*-tar mappings to both mapFileToPlatform() and getArchiveName(), keeping the legacy darwin-* cases for backwards compatibility

Root Cause

The files array in nodejs.org/dist/index.json uses the naming convention osx-x64-tar / osx-arm64-tar for macOS tarballs. The mirror script's switch statement only matched darwin-x64 / darwin-arm64, which aren't used in the index. The actual archive filenames on disk still use darwin- (e.g., node-v22.22.0-darwin-arm64.tar.gz), so getArchiveName() correctly maps osx-arm64-tar to the darwin-arm64 filename.

Impact

After merging and running the mirror sync workflow, this will populate darwin-arm64 and darwin-amd64 builds for all Node.js versions v15+, fixing:

• 493+ versions currently missing macOS builds in the manifest
• Integration test failures for "Migrate Node.js from System (macOS)"

Closes #223

Test plan

• Verify go build ./... passes in scripts/mirror-binaries/
• Verify go vet ./... passes
• After merge, run mirror-sync workflow with --sync-only to populate missing darwin binaries
• Run manifest generation to update embedded manifests
• Re-run integration tests to confirm macOS migration passes