Publish reusable frontend state and reactivity guidance with ECS examples

## Problem

Review guidance about Vue reactivity, store authority, derived state, and renderer side effects is scattered across ECS PR threads. The same mistakes recur because there is no use-case-to-idiomatic-pattern reference.

## Goal

Turn the ECS review cookbook into reusable frontend architecture guidance.

## Proposed Solution

Publish a state/reactivity guide mapping common use cases to the preferred Pinia, Vue, and ECS patterns, with counterexamples and focused ECS examples.

## Acceptance Criteria

- Covers authoritative state, derived state, reactive shells, explicit watcher dependencies, and renderer side effects.
- Includes a use-case-to-pattern table and ECS examples from current main.
- Identifies compatibility exceptions separately from preferred new code.
- Is linked from contributor guidance used during review.