Build Pi from the nixpkgs release package

[RestartDK]

Summary

• build Pi from the versioned nixpkgs-unstable release package
• retain the local compaction-context patch through overrideAttrs
• remove the direct pi-src input that tracked arbitrary upstream main commits
• use the dotfiles nixpkgs-unstable pin for the Cobb/network-namespace wrapper too

Why

Upstream Pi main at 5124c61 and 8b93737 reports version 0.80.10, but its generated provider TypeScript imports src/providers/data/*.json files that are not committed. The old derivation intentionally skips the networked model-generation step, so those development commits cannot build offline.

The actual v0.80.10 release source used by nixpkgs contains the generated model data inline and builds successfully. Tracking the nixpkgs release package avoids unreleased, internally incomplete snapshots while keeping Pi on 0.80.10.

Validation

• nix build --no-link for the overridden Pi package on aarch64-darwin
• clean x86_64-linux package build on titan-2
• Pi reports 0.80.10
• verified the compaction patch in the built session-manager.js
• full dkumlin-twin-macbook-pro Darwin toplevel build
• ./bin/traitor check --all-systems --no-build
• Cobb titan and titan-2 toplevel evaluation with this branch overridden