UW Markdown transport bindings

These optional companion profiles carry registered UW Markdown representations without changing the authoring format or conformance tiers.

• HTTP Binding 1.0 — content negotiation, semantic ETags, preconditions, statuses, limits, and deployment boundaries.
• MCP Binding 1.0 — stable resources, text/blob content, five uwmd.* tool profiles, structured results, and resource links.
• OpenAPI 3.1 contract — example paths, components, request bodies, and negotiated responses.

The runnable reference adapters delegate all serialization, validation, and editing to @uwmd/core.