Log the `claude-opus-4-7` design session that produced the phased plan
(A: modularize `_spawn`, B: `_subint` backend, C: harness) and concrete
Phase A file-split for #379. Substantive bc the plan directly drives
upcoming impl.
Prompt-IO: ai/prompt-io/claude/20260417T034918Z_9703210_prompt_io.md
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi