Standardize documentation and warnings for compatibility-driven invariant exceptions

## Problem

ECS reviews sometimes accept behavior that violates the preferred invariant for compatibility. These exceptions are inconsistently documented and may fail silently, making the decision model and eventual removal path unclear.

## Goal

Make every compatibility-driven invariant exception explicit, observable, and removable.

## Proposed Solution

Define a repository policy for recording the preferred invariant, the bounded exception, warning/telemetry behavior, tests, owner, and sunset. Route deprecations through the standard warning channel.

## Acceptance Criteria

- Contributor guidance defines the required record for a compatibility exception.
- New exceptions require a legacy test and a preferred-behavior test or explicit rationale.
- User/extension-visible deprecated behavior uses the standard deduplicated warning path.
- Existing ECS exceptions are inventoried with owners and sunsets.