Mac mini disk/CPU is saturated with 10+ concurrent PR builds.
Temporarily switching to GitHub-hosted macos-15 runners until we have
more disk space. Self-hosted was faster due to incremental Rust builds
but was causing CI queue buildup and false test failures.
Changes:
- CI: self-hosted → macos-15
- Release build: self-hosted → macos-15
- Remove per-runner pnpm store isolation (not needed on GitHub runners)
- Add Rust cache for GitHub runners (keyed by Cargo.lock)
- Remove CARGO_INCREMENTAL=1 (managed via cache instead)
c4c806c9a8