Add MASS4.Graph.Dataflow: pull evaluation, caching, acyclicity and coercion #13

Current State

Shader graphs, material graphs, compositors and data pipelines are pull-based dataflow: outputs are computed on demand from inputs, results are cached until inputs change, and cycles are invalid. None of that belongs in the core, which permits cycles.

Tasks:

• Pull evaluation order from requested outputs
• Per-instance cache slots and invalidation declared through the instance layout contract
• Acyclicity validation rule reporting the elements of each cycle
• Coercion and conversion strategy built on the core compatibility contract
• Wildcard port type inference from connected ports
• Evaluation-context hook that media domains extend with time and streaming

Acceptance Criteria:

• Cycles in a Dataflow document produce diagnostics naming the cycle's edges
• Unchanged inputs reuse cached results without reallocation
• Wildcard ports resolve to concrete types or report ambiguity
• Depends only on MASS4.Graph

Related:

• Epic: semantics triad — Dataflow, Control and State #2 (semantics epic), Prove semantics composition with a mixed State × Dataflow document #16 (mixed-document acceptance), Add the shader/material graph contract sample #21 (shader sample), Add the video compositor contract sample without engine references #23 (compositor sample)