Define package-detection and target-test-shape rules for generated tests

[cwinland]

Docs reference: docs/roadmap/generator-roadmap.md

Generated tests must be package-aware. This issue defines the shared package-detection and target-test-shape matrix that both generators and analyzers use.

Current dependency note:

• #125 is the settled constructor-selection and graph-metadata contract boundary that generated test shapes must target.
• This issue should decide package-aware harness or scaffold shape around that contract, not reopen the public construction-planning surface, parameter metadata model, or parameter-source enum from #125.
• Broader project or suite preferences about desired generated test direction belong here or later authoring-flow issues, not in #125.

Current implementation status:

• implemented on feature/122-graph-metadata-slice in commit c159ed8
• added a shared analyzer package-layout and target-test-shape matrix for aggregate, core-only, and split helper-package layouts
• wired MissingHelperPackageAnalyzer to the shared matrix so package guidance stops guessing from ad hoc checks
• added focused analyzer tests for core-only, split-helper, and aggregate layouts
• validated with dotnet test FastMoq.Analyzers.Tests/FastMoq.Analyzers.Tests.csproj across net8.0, net9.0, and net10.0

Acceptance checklist

• The package-detection matrix is explicit for aggregate, core-only, and split helper-package layouts.
• Supported generated test shapes are explicit for core, web, database, Azure, and Azure Functions package combinations.
• Generated imports, harness selection, scenario or scaffold shapes, and helper availability are all derived from the same documented rules.
• Analyzer package-suggestion and stop rules align with the same matrix before generation is offered.
• The issue stays separate from helper-surface normalization so package rules do not silently absorb API-shape design.
• Generated output never assumes an unreferenced helper package or ambiguous helper family.
• The issue does not restate or reopen the constructor-selection contract already defined in #125.

Follow-on boundaries

• helper-surface normalization stays in #134
• analyzer rollout stays in #124

[cwinland]

Implemented on feature/122-graph-metadata-slice in commit c159ed8 and tracked in PR #161. This lands the shared analyzer package-layout and target-test-shape matrix, wires MissingHelperPackageAnalyzer to it, adds focused layout coverage for core-only, split-helper, and aggregate package sets, and passes dotnet test FastMoq.Analyzers.Tests/FastMoq.Analyzers.Tests.csproj across net8.0, net9.0, and net10.0.