Workspace-Estimation Shared Header: DLL-Boundary Pitfalls

Lessons from implementing the two-level (EstimateWorkspace / DeclareWorkspaceRequirements) workspace-size estimation pilot for MatMulNBits (issue #29775 Phase-A, PR #29811). The WorkspaceRequirement struct in include/onnxruntime/core/framework/workspace_requirement.h is designed to be included by both in-tree kernel code and future plugin-EP adapter code — this is exactly the kind of dual-included, DLL-boundary-crossing header where these gotchas apply.

1. Never forward-declare Node in a header included from both worlds

Symptom: a compile failure or, worse, a silent type mismatch that depends on include order — because onnxruntime::Node is not one type across the whole codebase. In-tree code sees class Node from core/graph/graph.h. Shared-provider/plugin-bridge code (anything reachable from a plugin DLL) sees a different Node type from a provider-bridge header (e.g. struct Node final). These are two distinct types that happen to share a name — different "ODR worlds."