RFC: TypeSpec Breaking Change Detection Tool - Design Overview

Design overview for @azure-tools/typespec-breaking-change, a TypeSpec-native
API diff and breaking change detection tool for Azure ARM APIs.

Covers architecture, comparison phases, rule summary tables, suppression
mechanism, CI integration, and output format.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: markcowl