Add linear systems message-passing tutorial

[bvdmitri]

Summary

• Add an Advanced Example showing how to solve a small linear system with the Bilinear node.
• Compare exact/tree and loopy behavior, convergence, residuals, variances, and coupling strength with plots.
• Make examples-dev follow a local RxInfer checkout's local ReactiveMP path for development builds.

Verification

Built successfully with the Makefile command:

make examples-dev FILTER=Solving RXINFER=/Users/bvdmitri/Projects/Julia/RxInfer.jl

Dependency note

This example depends on the new Bilinear node and will not build against the currently released ReactiveMP package. It will build in CI and users' environments once a new ReactiveMP release containing that node is published; local development was verified using the local RxInfer and ReactiveMP checkouts.

@Nimrais — please review, especially the tutorial framing and whether the node should be renamed before release.