rune compile back-end — background compilation and transparent hot-swap.
Architecture:
- CompileQueue: background thread pool for compilation jobs
- ArtifactCache: source-hash → compiled artifact mapping
- TierRouter: per-call-site tier (interpret | compile | compiled)
- hot-swap: when compile finishes, next call uses compiled path
The interpreter is always the fallback. Compilation is opportunistic. Instant start is always preserved — compilation never blocks execution.