docs: FileDataAdapter recipe (ADR-0095 fulfillment)

[SisyphusZheng]

Summary

Provide a documented FileDataAdapter recipe showing build-time data loading from JSON/YAML files, fulfilling ADR-0095's recipe promise. The framework core must not import node:fs — this is a user-space pattern.

Competitive context

No mainstream framework bundles a file-data adapter internally. Astro uses content collections (build-time); SvelteKit/Next.js leave data loading to user code. openElement's ADR-0095 correctly defers this to recipe level but provides zero examples.

Proposed deliverable

1. Recipe doc (docs/integrations/file-data-adapter.md):

import { readFileSync } from 'node:fs';
import type { DataAdapter } from '@openelement/element';

export class FileDataAdapter<T> implements DataAdapter<T> {
  readonly name = 'file';
  #dir: string;

  constructor(dir: string) { this.#dir = dir; }

  async get(key: string): Promise<T | undefined> {
    try {
      const raw = readFileSync(join(this.#dir, `${key}.json`), 'utf-8');
      return JSON.parse(raw) as T;
    } catch { return undefined; }
  }

  async keys(): Promise<string[]> {
    return readdirSync(this.#dir)
      .filter(f => f.endsWith('.json'))
      .map(f => f.replace('.json', ''));
  }
}

2. Usage in loader:

const db = new FileDataAdapter<Product>('./data/products');

export const loader = async ({ params }) => {
  return { product: await db.get(params.id) };
};

Non-goals

• No @openelement/file-adapter package
• No watch/hot-reload (build-time only)
• No database migration

Target version

0.43.0 (documentation)