built_project/project_lock/ScratchPath/ScratchGuard move from their three
per-binary copies (cli_run.rs, research_docs.rs, project_load.rs's
built_fixture near-copy) into tests/common/mod.rs; project_load's one
shared-store-mutating test now takes the lock it previously lacked. The
module doc states the boundary plainly: the Mutex serializes threads
within one process only — process-parallel runners still race on the
shared fixture store (documented, dormant under cargo test's sequential
binaries; see the issue thread for the observed process-parallel repro).
closes#223
