Survive broken pipes, add loom transcripts, attribute child cast results

[deepfates]

Three fixes to the cast runtime and renderer.

What changed

• A broken pipe on stdout no longer crashes a cast. The loom is flushed and closed on the way down, and the process exits with a shutdown tuple instead of a crash.
• The loom can produce a readable transcript, and turn inspection is bounded instead of dumping everything.
• Tool results from child casts are attributed to the child that produced them, at the port layer, so the renderer shows real provenance instead of generic labels.

Testing

• mix test passes, including roughly 200 lines of new renderer tests.
• Attribution verified by running a parent cast that delegates to a child and reading the rendered output.