Add `open_wg_bindspace()` to enter one declared bindspace and an
ordered WireGuard interface stack as one async lifetime.
Deats,
- snapshot caller layer ordering before the first checkpoint
- enter interfaces outermost-first through `AsyncExitStack`
- unwind interfaces before releasing the namespace capability
- yield the live `BindspaceHandle` for endpoint allocation
- test mutable input and cancellation ordering with lifecycle fakes
Prompt-IO: ai/prompt-io/opencode/20260826T022434Z_2245f094_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi