feat: add proto plugin for typed data contracts

[keugenek]

Summary

New AppKit plugin that provides protobuf-based serialization for typed data contracts across plugins, routes, and jobs. Consumers bring their own .proto files — AppKit provides the serialization layer.

Plugin API

Method	Description
create(schema, init?)	Create a typed proto message
serialize(schema, msg)	Serialize to binary (Uint8Array)
deserialize(schema, data)	Deserialize from binary
toJSON(schema, msg)	Convert to JSON (proto field names)
fromJSON(schema, json)	Parse from JSON

Usage

import { createApp, proto, files, server } from "@databricks/appkit";

const app = await createApp({
  plugins: [server(), proto(), files()],
});

const customer = app.proto.create(CustomerSchema, {
  id: "cust-001",
  name: "Acme Corp",
});

// Binary for storage — pair with Files plugin
const bytes = app.proto.serialize(CustomerSchema, customer);
await app.files("data").upload("customer.bin", Buffer.from(bytes));

// JSON for APIs
res.json(app.proto.toJSON(CustomerSchema, customer));

What's included

• Proto plugin: packages/appkit/src/plugins/proto/ (plugin, serializer, types, manifest)
• Unit tests: plugin + serializer
• E2E scenario tests: Product Catalog sample app with Playwright
• Documentation: docs/docs/plugins/proto.md

Design decisions

• Pure serialization — no file I/O. Pair with Files plugin for Volume access, Lakebase plugin for database storage.
• Zero config — proto() with no arguments.
• No bundled proto definitions — consumers bring their own .proto files and generate their own types. AppKit doesn't couple to any domain vocabulary.

Test plan

• Plugin unit tests pass (5/5)
• Serializer unit tests pass (6/6) — round-trip, nested objects, JSON, error handling
• Exports only serialization API — no file I/O methods (writeToVolume, readFromVolume, exists are absent)
• E2E scenario tests pass (13/13) — Product Catalog sample app
  • 5 public: view all, filter by category, health check, snake_case fields, API validation
  • 8 private: single-category, in/out-of-stock, product detail, 404, proto content-type, status text, column headers
  • Tested locally: npx tsx app/server.ts + npx playwright test
• pnpm build passes
• pnpm test passes (CI)