Add oxalica/rust-overlay flake input; override rustPlatform in
weft-packages.nix with cargo/rustc from rust-bin.stable.1.93.0;
apply overlay in both devShell and nixosConfigurations.weft-vm.
Fixes build failure where nixos-25.05 Rust 1.86.0 was below the
rust-version requirement of wasmtime and other dependencies.
- flake.nix, infra/nixos/: NixOS VM with Mesa, virtio-gpu, Wayland,
systemd user services for compositor and session supervisor
- infra/vm/: QEMU build and run scripts
- .github/workflows/ci.yml: add Linux job to type-check weft-servo-shell
and weft-app-shell with --features servo-embed
- docs/architecture.md, docs/security.md, docs/building.md: replace
stale pre-implementation design documents
- README.md: rewrite to reflect current codebase
- crates/weft-servo-shell/SERVO_PIN.md: update implementation status and add
SpiderMonkey process boundary statement