fix: Isolate Cargo cache by host platform (#13337)
## Why
Cargo task hashes included the compiler release but not its host
platform. Identical source inputs could therefore restore incompatible
native artifacts across operating systems, architectures, or host ABIs.
## What
Cargo package identities now include the complete normalized `rustc -vV`
output, including the host triple. Discovery fails closed when that
identity cannot be resolved, while memberless workspaces remain
unaffected.
## How
Added parser, host-collision, descriptor, and memberless-workspace
coverage. Verified all 311 `turborepo-repository` tests, `cargo lint`,
Rust formatting, and pre-push hooks.
