Thread an optional live `Bindspace` through actor spawn APIs and
give each Trio exec child an inherited namespace descriptor before
runtime bootstrap.
Deats,
- duplicate the namespace FD without changing parent ownership
- preserve caller `pass_fds` and process options
- send the `(fd, inode)` pair through the `_child` CLI
- close the parent duplicate on success, failure and cancellation
- reject MP backends until descriptor reduction is implemented
- avoid cleanup before child publication
- exercise real E2E namespace relay through handshake and RPC
- verify parent FD-table cleanup after successful child spawn
Prompt-IO: ai/prompt-io/opencode/20260828T172943Z_2ca8c570_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi