Coarse-graining support: Support.Book.Ch03.Theorems.PublicInternalBridges

Imported from the Apache-2.0 CoarseGraining development at commit c7ddd76c08ade64fed1b8d2ca51be14dfee8deb4.

Public/internal bridges for Chapter 3

This file is the stable import surface for the Chapter 3 public/internal bridge endpoints. The proof bodies live in focused PublicInternalBridges/ submodules so downstream files can keep importing this module without pulling a monolithic source file into the edit loop.