Update examples to platform 0.9 and refresh CI

[Anton-4]

Summary

• Update the example platform URLs to lukewilliamboswell/roc-platform-template-zig release 0.9.
• Adjust examples/headers.roc for the current Roc API by using Str.join_with(...) directly.
• Refresh CI for the current nightly compiler path and bundle flow, including workspace-local temp paths for copied bundle examples.
• Keep macOS coverage on macOS 15, using the macos-15-intel runner because arm64 macos-15/macos-latest runner allocation stayed queued.

Testing

• roc fmt --check package examples
• roc check package/main.roc
• roc check examples/headers.roc --no-cache
• ci/all_tests.sh
• GitHub Actions: Ubuntu and Windows package/bundle checks passed on the previous pushed commit; this push moves macOS coverage to macos-15-intel to unblock hosted runner scheduling.