plan(vm): define invocation item stream contract

[fffonion]

Summary

• define an invocation as ordered event values followed by one explicit terminal item
• specify one-slot pull backpressure, cancellation ownership, and typed failure preservation

Stack

Layer 10 of 14
Previous: #20
Next: #23

Validation

• cargo fmt --all -- --check
• cargo test --locked --workspace
• cargo test --locked --workspace --all-features
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings

[fffonion]

Superseded by #14 and #23. The plan moved to the canonical roadmap; the implementation moved to the invocation-stream layer.