Document extended NM client architecture

[osirvent]

Summary

• add the extended-client context, architecture, NM 27.0 contract guide, and implementation roadmap
• document the curated, backward-compatible approach for expanding NM B2B coverage
• direct future extension work to this context from AGENTS.md

Why

The current client intentionally implements only part of the NM B2B surface. These documents turn the supplied NM 27.0 WSDL/XSD, examples, reference manuals, and existing client structure into an implementation-ready plan for broader coverage.

Impact

No runtime behavior or public API changes. Future contributors have an authoritative contract strategy, complete operation inventory, compatibility rules, and staged rollout plan.

Validation

• git diff --check
• verified the WSDL operation matrix contains all 197 operations, with the 24 currently supported operations classified separately
• verified local Markdown links resolve

The package test commands were previously blocked because dependencies are not installed in this checkout; no source code changed in this PR.

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