Drop NixCI dependency edges to doNotBuild jobs ## Problem The newer NixCI validates build-dependency edges and rejects any edge whose endpoint is a job it does not build (i.e. a job listed in `doNotBuild`). The manual dependency graph in `nix-ci.nix` referenced **35** such jobs, so NixCI now shows a `build-dependency configuration error`: - `checks.x86_64-linux.gammaloop-doctest`, `checks.x86_64-linux.gammaloop-nextest-binaries` - every `crate-*` and `crate-deps-*` artifact (all `doNotBuild`) - `packages.x86_64-linux.default`, `workspaceBuildArtifacts`, `workspaceDoctestCargoArtifacts`, `gammaloop-llvm-coverage`, `nix-ci-check-gammaloop-nextest` These jobs exist in the flake but are excluded from CI via `doNotBuild`, so ordering edges to/from them are meaningless — NixCI never schedules them. ## Fix Keep constructing the full crate/artifact DAG (so the drift and cycle asserts stay meaningful), then run the emitted graph through a new `buildableDependencies` filter that drops every edge touching a `doNotBuild` job. Only ordering between jobs NixCI actually builds survives. Result: **162** meaningless edges dropped, **96** edges between built jobs kept (workspace-hack cache root, the test-support DAG, and nextest-archive ordering — all correctness-neutral scheduling hints). ## Testing - `nix eval` of `nix-ci.nix` succeeds. - An assertion over the emitted graph confirms **0** remaining edges reference a `doNotBuild` job (down from 35 endpoints / 162 edges).