[octalide]

Closes #4849

The snapshot owns copies of what its loaded project keeps past project_open: the artifact, target and profile strings (a ProjKey, shared with the loaded-project path through proj_key_capture and proj_key_free, which now take an allocator) and the root. driver.begin_build_impl sets p.req.project_root = project_root, and root_analyze sets rooted.project_root = root, so the project held the caller's req.root. The request is now built from the copies, all freed in project_dnit. The buffer array and buffer paths stay borrowed only during the call.

Ran: bootstrap.sh build, mach check . and mach fmt --check . clean. No new tests.

Decisions

• key is a private ProjKey and root a plain owned str beside it. The loaded-project path already owns its root separately, so the root is not folded into ProjKey. No new pub declaration.
• The copies are made in project_open before the load and freed in project_dnit after the project.