Move torus-node build into the flake and pin nixpkgs to 25.11

[steinerkelvin]

Torusform currently has to reconstruct the torus-substrate toolchain and build environment locally.

The upstream flake-style build path works with a plain cargo build inside the original flake/dev environment, but the downstream pure package still needs Nix-specific shims around vendored compiler_builtins / wbuild.

Requested change:

• Move the torus-node build/package into the torus-substrate flake.
• Pin nixpkgs there to 25.11.
• Expose a reusable package output for downstream repos.

Acceptance criteria:

• torus-substrate flake is pinned to nixos-25.11
• the flake exposes a torus-node package output
• torusform stops recreating the toolchain and cargo environment locally