WASM resident host is now default-on.

The compile-once/instantiate-many resident host (per-instance egress isolation, migrate-on-expose, idle-TTL reaper) is enabled by default: non-listen wasm creates instantiate into a shared resident host (~ms) instead of paying the ~2.8s cold CompileModule. Live-validated on cluster-3-mixed-wasm (v0.7.12): resident path only, server p50 22ms / p99 50ms, 0 failures, the ~3s cold-compile tail eliminated.

• SB_WASM_RESIDENT_HOST_ENABLED now defaults to true. Set it to false to force the per-sandbox worker path. Owner-scoped (ownerRef,digest,memoryMB) buckets keep different customers in separate processes.
• Warm pool auto-skipped when the resident host is on (they're redundant; running both double-consumed RAM and starved small hosts).
• SB_WASM_RESIDENT_HOST_MAX_INSTANCES (default 32, spill) + SB_WASM_RESIDENT_HOST_IDLE_TTL (default 5m) tune capacity/lifecycle.

Full Changelog: v0.7.12...v0.7.13