[TypeSpec Authoring] Align downstream SDK breaking-change assessment rules with SDK Review Board

## Parent issue

Part of #14529.

## Problem

The downstream SDK breaking-change rules used by the assessment skill were initially authored without a confirmed SDK Review Board source of truth.

## Goal

Validate and align the assessment rules with SDK Review Board policy and documented language-specific compatibility guidance.

## Scope

- Start with Johnathon to identify existing policy, design notes, rule catalogs, and the appropriate reviewers.
- Review the current assessment rules with SDK Review Board stakeholders.
- Distinguish cross-language rules from language-specific rules and document precedence.
- Resolve conflicts, unsupported assumptions, and gaps in the current rules.
- Add traceable citations or provenance for every enforced rule.
- Define a process for keeping rules synchronized when board guidance changes.

## Acceptance criteria

- The relevant SDK Review Board stakeholders approve the rule set or record explicit open questions.
- Every downstream breaking-change rule has a documented source, owner, and applicability.
- Conflicting or speculative rules are corrected, removed, or clearly marked as provisional.
- Benchmark coverage is updated for materially changed rules.

Start with Johnathon to locate the existing assessment rules, policy, design notes, and rule catalogs, then identify the appropriate SDK Review Board stakeholders. Compare the rules with approved cross-language and language-specific guidance, record sources, owners, applicability, and open questions, and update benchmark coverage for materially changed rules.

Written by the indexing model from the issue text.