The salvaged stack added three tests for response framing. Keep the
writer->reader round trip with nested frames (red on main: rpartition
injects only the embedded subsection) and the truncated-outer-frame
case. Drop the test and the in-test block that exercise "framed archive
without a prompt length": that shape was only ever produced by the PR's
own intermediate commits, never by a writer on main, and the follow-up
removes the reader branch for it.