docs: define test project-reference boundaries

[SunSi12138]

Motivation

Issue #431 split test-originating ProjectReference topology out of the production-only boundary work from #371. The repository has a non-trivial test graph (including shared contracts/plugins, package-rooting projects, AOT artifacts, load-test support, and fixture projects) but no canonical policy describing which direct edges and modes are intentional.

Closes #431.

Changes

• Add doc/test-project-reference-boundaries.yml as a machine-readable, default-deny policy for ProjectReference edges originating under test/.
• Inventory the current dev graph at 3e772ef0de75bda9f79d5f3508ef01f573bb6254: 107 direct test-originating edges.
• Classify normal test-to-production edges as allowed and test-to-test/support edges as intentional_test_infrastructure; there are currently no debt exceptions.
• Define exact assembly, analyzer, and build_only mode semantics, including ReferenceOutputAssembly=false build/order-only dependencies.
• Cover solution and non-solution fixtures, including MultiCluster contracts, RollbackPlugin, StaticCodecOwner contracts, package-rooting support, and the generated API4 fixture.
• Keep doc/project-reference-boundaries.yml authoritative for all src/ origins; this policy cannot relax production rules.
• Add the new boundary document to doc/index.md.
• Do not add or change CI enforcement.

Compatibility

• Public API: no change.
• Protocol/wire format: no change.
• Generated code or contract manifest: no change.
• NativeAOT and platform behavior: no change.

Validation

• Compared the policy against every current top-level test/*.csproj project and the nested test/fixtures project set on the baseline commit.
• Verified the grouped policy expands to 107 direct ProjectReference edges.
• Verified current reference metadata falls into the documented assembly, analyzer, or build_only modes.
• Verified the branch changes only documentation/policy files and does not modify CI enforcement.
• Non-incremental Release build: not run; this is a documentation/design-only PR with no build inputs changed.
• Runtime/unit/integration tests: not run for the same reason.

Evidence

• Production authority: doc/project-reference-boundaries.yml from [Maintainability][Architecture] Document the intended production project dependency graph #371.
• Test policy: doc/test-project-reference-boundaries.yml.
• Human-readable inventory and mechanical interpretation: doc/test-project-reference-boundaries.md.
• Baseline: dev@3e772ef0de75bda9f79d5f3508ef01f573bb6254.
• Diff contains only doc/index.md plus the two new test-boundary documents.