Add Composite tree schema validation — DP063/DP064/DP065

[Skymly]

Summary

• Add [CompositeSchema(MaxDepth, MaxNodes)] attribute for contract-level tree structure constraints
• Add [CompositePart(AllowedChildTypes)] property for per-node child type compatibility enforcement
• Add DP063 (max depth exceeded, Warning), DP064 (child type not allowed, Error), DP065 (node count exceeded, Warning)
• All constraints are opt-in — unmarked contracts behave as before, zero breaking changes
• Zero runtime overhead — all validation in source generator
• RFC: docs/rfc/CompositeTreeSchemaValidation.md
• Closes Composite tree schema validation — compile-time max depth / parent-child type compatibility / node count #218

Changes

• Runtime: CompositeSchemaAttribute.cs (new), CompositePartAttribute.cs + TContract.cs (AllowedChildTypes)
• Diagnostics: DiagnosticIds.cs + DesignPatternsDiagnosticDescriptors.cs (DP063-DP065)
• SourceGenerators: CompositePartGenerator.cs (schema extraction, depth computation, type validation, node count)
• Tests: 10 new generator tests covering all 3 diagnostics + negative cases + forest mode + non-generic attribute
• Docs: ROADMAP.md, AGENTS.md, RFC

Test plan

• dotnet build DesignPatterns.slnx -c Release — zero errors, zero warnings
• dotnet test DesignPatterns.slnx -c Release — 651 tests pass (146 SourceGenerators, 324 Runtime, 80 Analyzers, 71 DI, 14 Autofac, 16 Config/AppSettings)
• 10 new tests: ReportsDp063MaxDepthExceeded, ReportsDp063NotExceededWhenWithinLimit, ReportsDp063ForestModeComputesPerTree, ReportsDp064ChildTypeNotAllowed, ReportsDp064AllowedWhenTypeInList, ReportsDp064NotCheckedWhenAllowedChildTypesNull, ReportsDp064WithNonGenericAttribute, ReportsDp065NodeCountExceeded, ReportsDp065NotExceededWhenWithinLimit, NoSchemaAttributeNoNewDiagnostics
• CI passes