The harness assumed that the Rust interop peer builds under
packages/cultnet-rs/target. When CARGO_TARGET_DIR or a cargo config's
build.target-dir is set, cargo builds elsewhere, and every Rust lane
failed with a spawn ENOENT. The path now comes from `cargo metadata`'s
target_directory, read once on first use.
The TypeScript peer imports @gamecult/cultcache-ts, and the Python peers
import msgpack. When either was missing, the lanes died with import
errors in a child's stderr or with timeouts. The first use of each peer
now checks its prerequisite and fails with a message that names it and
the command that provides it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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