[M0] Repair clean-checkout build gates and pin a compatible Rust/Swift toolchain

Spec / invariant

Definition of done; FR-C10, FR-R07. A fresh clone must reproduce every generated native artifact and test result.

Current evidence

CI pins Rust 1.85 while Cargo.lock contains packages requiring newer compilers. Apple CI runs Swift tests without first producing the ignored universal XCFramework, so a clean runner cannot resolve NMPNativeRuntime.

Deliverables

Pin one toolchain compatible with the lock or make the lock compatible; build/regenerate UniFFI Swift sources and the universal XCFramework in CI; validate generated bindings are current; document clean-clone commands.

Acceptance

A fresh checkout with empty target/.build passes Rust fmt/clippy/tests, the Swift package tests, Apple package tests, and the shared Workbench scheme on both arm64 and x86_64 artifacts. No generated binary is committed.

Dependencies

None.

Global constraints

• ~/Work/nmp is read-only; do not modify it.
• Use the pinned NMP public facade only and keep queues/maps/frames finite.
• Do not advertise behavior until its contract and teardown tests pass.

Inspect the CI toolchain pins and Cargo.lock first, then trace the UniFFI Swift-source and universal XCFramework generation steps. Run the existing Rust, Swift, Apple package, and Workbench checks from an empty target/.build checkout on both arm64 and x86_64. Done means all gates pass without committing generated binaries and clean-clone commands are documented.