Add serialized `BindspaceSpec.lifecycle` policy and dispatch it through
`open_bindspace()` without inferring ownership from transport role.
Deats,
- distinguish borrowed `attach` from owned `open` policy
- validate handle ownership against the declared lifecycle
- share policy-neutral FD pinning between both netns contexts
- reject unsupported lifecycle values before side effects
- exercise both dispatcher branches and owned cancellation cleanup
Prompt-IO: ai/prompt-io/opencode/20260825T191845Z_5b2a064a_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi