Create RFQ_CASE_MATRIX.md

[gbl186]

Added a simple RFQ scenario matrix so contributors can see coverage gaps at a glance

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Note

Low Risk
Low risk documentation-only change with no impact on runtime behavior or security.

Overview
Adds docs/RFQ_CASE_MATRIX.md, a concise RFQ scenario matrix covering happy paths, failure paths, and network issues to make coverage expectations explicit.

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