🐛 Isolate VM lifecycle state between interpreter instances

[ccamel]

Summary

SetMaxVariables and ResetEnv currently mutate global quota, variable identity, root environment, and stream IDs through engine/vm.go:399-439, engine/variable.go:10-38, engine/env.go:1-46, and engine/stream.go:19-31. Interleaving otherwise independent interpreter instances can therefore change each other's lifecycle state.

Why it matters

Interpreter isolation is required for deterministic execution. A reset in one VM can invalidate assumptions held by another VM, including retained Env bindings and stream identities.

Scope

Move variable-quota and reset ownership into a VM/session allocator while preserving globally unique term identities. Do not use a lock-only fix. This is not a quick win; design and review it separately from the seven focused issues in this set.

Acceptance criteria

• Two interleaved VM instances retain independent variable limits.
• Resetting one VM cannot alias bindings in an Env retained by another VM.
• Resetting one VM cannot alias stream IDs retained by another VM.
• A focused VM-isolation test passes with -race.

Verification

Run the focused isolation test with go test -race and exercise two interleaved VMs with distinct limits, retained environments, and retained stream IDs.