[Epic][TypeSpec Authoring] Support Advanced Authoring Cases

## Overview

Extend the `azure-typespec-author` skill to support advanced TypeSpec authoring cases beyond the currently handled baseline scenarios (Add Resource Type, Add Resource Operations, API Version Evolution).

## Motivation

The current skill covers foundational ARM authoring patterns. As adoption grows, users encounter more complex real-world scenarios that the skill does not yet handle — such as virtual resource, custom LRO patterns, complex resource hierachy. This epic tracks the work to identify, design, and implement support for these advanced cases.

## Goals

- Identify the most impactful advanced authoring cases from user feedback and benchmark gaps
- Validate each case with benchmark tests before shipping

## Sub-Issues

Tracked as sub-issues below.