EdenFS OSS build and startup fail on macOS arm64 #1478

A clean EdenFS OSS build on macOS arm64 encounters multiple blockers:

• Rust build: The build requests Cargo package backingstore, but the package is named sapling-backingstore.
• C++ build: Compilation fails because the generated backingstore/src/ffi.rs.h is unavailable when needed and FuseDirList.h uses std::vector without including <vector>.
• Client packaging: The OSS build does not produce a usable Python edenfsctl with its required Thrift bindings.
• Daemon startup: When built with the macOS 27 SDK, the build selects pipe2. The resulting daemon crashes in Pipe::Pipe on macOS 26, where that API is unavailable.

Expected: The OSS build produces a daemon, privileged helper, and client that can start and mount an EdenFS checkout on macOS.

Related: #683 discusses earlier external EdenFS setup attempts. The Sapling CLI dependency failure in #1476 is separate.