docs(furniture-bff): add service boundary notes

docs/architecture/bff-service-boundary.md

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+# Furniture BFF Service Boundary
+
+## Purpose
+Preserve BFF as an edge adapter layer that depends on service contracts only.
+
+## BFF Responsibilities
+- REST edge exposure
+- Service client adaptation
+- Correlation/tracing propagation
+
+## Prohibited
+- Direct DAL access
+- Domain business decision ownership

docs/migration/correlation-propagation-checks.md

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+# Correlation Propagation Checks
+
+## Checks
+- Correlation identifiers are preserved across BFF -> Service calls.
+- No new correlation behavior is introduced at edge.
+- Trace metadata pass-through remains stable.

docs/migration/post-domain-contract-alignment.md

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+# Post-Domain Contract Alignment
+
+## Goal
+Align BFF adapter usage with service contracts after domain extraction.
+
+## Steps
+1. Map current BFF contract usage to updated service contracts.
+2. Keep edge contract behavior stable.
+3. Validate adapter compatibility.