Composite tree schema validation — compile-time max depth / parent-child type compatibility / node count

[Skymly]

Motivation

ROADMAP long-term exploration candidate: Composite tree schema validation.

Existing Composite diagnostics (DP010-DP015, DP040-DP041) cover key uniqueness, parent key existence, cycle detection, contract implementation, constructor, DI registration — but do not validate the overall tree structure shape. This feature adds compile-time validation for max depth, parent-child type compatibility, and node count.

Design

RFC: docs/rfc/CompositeTreeSchemaValidation.md

API

• [CompositeSchema(MaxDepth, MaxNodes)] on contract interface — global tree constraints
• [CompositePart(AllowedChildTypes)] on parent nodes — per-node child type restriction

Diagnostics

• DP063: CompositeTreeMaxDepthExceeded (Warning)
• DP064: CompositeChildTypeNotAllowed (Error)
• DP065: CompositeNodeCountExceeded (Warning)

Backward compatibility

All constraints are opt-in. Unmarked contracts behave as before — zero breaking changes.