feat(vm): add invocation item stream

[fffonion]

Summary

• add an invocation item stream with explicit event, completion, and failure items
• enforce one-slot pull backpressure and consume cancellation at the stream boundary
• preserve typed host-operation failures and document runtime ownership semantics

Stack

Layer 11 of 13
Previous: #22
Next: #24

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