Modal moves serverless Functions to a regional I/O plane, cutting p50 latency by ~80ms

Modal has migrated all Functions to a new geographically distributed I/O plane, dropping end-to-end Function Call latency by roughly 80ms at p50. Previously every Function's inputs and outputs routed through us-east I/O servers, adding up to a few 100ms per call; the new plane is available in ≥4 regions and selectable via the routing_region flag. The server layer was rewritten in Go, with non-critical work moved off the hot path.