Isolate ECS link-store traversal and legacy output.links materialization cost

## Problem

The #15977 investigation identified link-store traversal and compatibility output.links materialization as plausible independent renderer costs, but did not isolate them after proving the redraw feedback loop. Fan-out and extension facades may amplify this path.

## Goal

Measure the independent cost and define a safe optimization boundary.

## Proposed Solution

Build fan-out and extension-compatibility fixtures, instrument link traversal and snapshot materialization, and compare current behavior with isolated alternatives while preserving topology authority.

## Acceptance Criteria

- Fixtures cover core fan-out and compatibility output.links consumers.
- Traversal/materialization counts and task cost are measured independently of redraw count.
- Any optimization preserves link identity, mutation compatibility, serialization, and undo behavior.
- Results end in a bounded fix or an evidence-backed no-action decision.