Document ECS migration paths for extension mutation and serialization APIs

## Problem

The ECS migration documentation covers link registration but does not give extension authors actionable migrations for direct slot-array mutation, node replacement, promoted widgets, serialization hooks, and geometry access. Those are the compatibility categories most likely to keep extensions on legacy mutation paths.

## Goal

Give extension authors one supported migration map from legacy behavior to the post-ECS APIs.

## Proposed Solution

Add an extension-facing guide organized by legacy use case, supported replacement, compatibility lifetime, and runnable example. Link it from the stable API and ECS documentation.

## Acceptance Criteria

- Direct slot-array mutation, node replacement, promoted widgets, serialization hooks, and geometry each have a supported replacement or an explicit gap.
- Examples use public APIs and include migration notes for existing extensions.
- Compatibility-only behavior is labelled with its deprecation/removal policy.
- Documentation is checked against current main and reviewed by the ECS owner.