Add private `enter_netns()` bootstrap validation before actor runtime
integration.
Deats,
- verify the inherited FD against its expected namespace inode
- constrain `setns()` to Linux network namespaces
- verify `/proc/self/ns/net` after the entry syscall
- leave inherited FD ownership to the future spawn caller
- exercise guards with real FDs and unprivileged syscall fakes
Prompt-IO: ai/prompt-io/opencode/20260827T232500Z_d067505a_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi