Add an ArchUnitNET architecture-fitness suite with a ratcheting baseline

[ChrisonSimtian]

What

Adds tests/Fallout.Architecture.Tests — a solution-wide ArchUnitNET suite that codifies Fallout's intended runtime architecture so future work can't silently invert a dependency edge that still compiles. This is the broader-suite half of Fallout-build#95, and it absorbs the Fallout-build#88 Fallout.Core purity guard.

15 tests, all green. 14 are strict invariants that hold today; 1 — the namespace/assembly-alignment rule — carries a documented baseline of the current legacy debt.

Key decisions

• ArchUnitNET, not the existing NetArchTest.Rules. NetArchTest is effectively unmaintained (last release 2023); ArchUnitNET is maintained and far more expressive. The [Foundation] Extract Fallout.Core domain project Fallout-build/Fallout#88 Fallout.Core guard was migrated onto it and NetArchTest.Rules was retired from Directory.Packages.props.
• Rules are scoped by assembly, not namespace. The legacy NUKE namespaces deliberately span assemblies (Fallout.Common.* types live in Fallout.Build, Fallout.Build.Shared and Fallout.Common; the Fallout.Utilities.Net assembly still emits Fallout.Common.Utilities.Net.*), so namespace-based layering would be meaningless. Every subject/target uses ResideInAssembly(...), additionally filtered to first-party (Fallout.* / Nuke.*) types to strip tooling-generated noise (ThisAssembly, RefSafetyRulesAttribute, coverlet).
• A one-way ratchet for known-broken areas. The architecture is partially broken by design right now, so rather than fail the build on day-one debt, each rule is allowed exactly its documented baseline (KnownViolations). The test fails on any new violation and on any baseline entry that has been fixed — so the debt register can only shrink.

Enforced strictly (empty baseline — these hold today)

• Fallout.Core and Fallout.Utilities are foundations: no in-repo dependencies.
• Each Fallout.Utilities.* satellite depends only on Fallout.Utilities.
• Fallout.Tooling, Fallout.ProjectModel, Fallout.Build.Shared don't reach upward; Fallout.Solution is a thin facade over the vendored parser.
• Nothing depends on the Fallout.Cli composition root.
• Fallout.* never depends on the Nuke.* transition shims (they point inward only).
• Fallout.Core takes no dependency on System.IO / System.Diagnostics.Process / System.Console / Serilog (the [Foundation] Extract Fallout.Core domain project Fallout-build/Fallout#88 purity bar).

Baselined (known debt, ratcheting down)

• Namespace ↔ assembly drift — a type's namespace should be rooted at its assembly name. Ships with a ~220-entry baseline capturing today's Fallout.Common.* sprawl (chiefly under Fallout.Build/Common/Tooling), plus the satellites still emitting Fallout.Common.Utilities.* and the Fallout.Solutions.* facade types. Shrinks as the onion-architecture refactor renames things; each removed entry is a permanent gain the ratchet locks in.

Relationship to refactor/architecture

That branch already has an onion/ring architecture-test suite (NetArchTest, ring-based) targeting a structure not yet on main (Fallout.Domain, Fallout.Application.*, …). This PR is deliberately the fresh suite on main encoding today's assembly layering — independent of the refactor. When refactor/architecture lands, the two reconcile (ideally that suite also moves to ArchUnitNET so there's one tool).

Out of scope (not governed)

The Roslyn build-time tooling (Fallout.SourceGenerators, Fallout.Tooling.Generator, Fallout.Migrate[.Analyzers], Fallout.MSBuildTasks) and the vendored Fallout.Persistence.Solution parser — composition-root / build-time / vendored code outside the runtime layering.

Notes

• Non-breaking; targets main. No version.json change.
• The target/2026 label didn't exist on this repo — created it to follow the PR-creation flow.
• How the suite works, and how to add a rule / regenerate the baseline: docs/architecture-tests.md.

Passed!  -  Failed: 0, Passed: 15, Skipped: 0, Total: 15  -  Fallout.Architecture.Tests.dll (net10.0)

🤖 Generated with Claude Code

[ChrisonSimtian]

Folding this work into the two onion-architecture PRs rather than landing it standalone:

• Move pure build-execution domain enums into Fallout.Core #9 now carries the full ArchUnitNET suite + ratcheting baseline (it's the natural home — Fallout.Core purity is the headline rule, and this replaces the old NetArchTest guard that lived in Fallout.Core.Tests).
• Split the tool-execution contract into Fallout.Application.Tooling.Execution + .Requirements #10 (stacked on Move pure build-execution domain enums into Fallout.Core #9) extends the suite to govern the new Fallout.Tooling.Abstractions leaf.

Same suite, same ratchet, same baseline — just delivered alongside the refactors it enforces so the architecture work and its guardrails land together. Closing in favour of those.