feat(vm): expose cancellable invocation item streams

[fffonion]

Summary

• Exposes typed Event, Complete, and terminal error items through Invocation polling.
• Preserves deadline, fuel, cancellation, late-completion rejection, and immediate VM reuse after Drop.
• Folds later abandoned-invocation cleanup fixes into the owning implementation.

Stack

• Base: refactor(host): bind capabilities and async execution #18
• Next: feat(compiler): finalize frame-local typing and ownership #24

Verification

• Invocation stream: 18 passed
• Runtime context: 25 passed
• Runtime host: 4 passed
• Workspace all-features compile and strict Clippy passed

Supersedes

Implementation portion of #22.