Idea: `neon-instant` connector for dev-time database provisioning

[pi0x]

Follow-up to #191 , which lands the neon connector as SDK-only: it talks to @neondatabase/serverless and requires a connection string (or a host in the ClientConfig).

An implementation of a second, neon-instant connector was prototyped in that PR and dropped before merge to keep the scope to one connector. Opening this to track the idea (perior work (ffe8fe9).

Idea

A neon-instant connector that provisions a claimable Neon Postgres database on demand via neon-new, so a project can createDatabase(neonInstant()) with no connection string and no signup during development.

import { createDatabase } from "db0";
import neonInstant from "db0/connectors/neon-instant";

const db = createDatabase(neonInstant());

Resolution order on first use (the client connects lazily):

1. An explicit url / connectionString (or a host) — never provisions.
2. process.env.DATABASE_URL — reuses the database an earlier run provisioned.
3. Otherwise, provision via instantPostgres() from neon-new, optionally seeding from a .sql file.

Design notes from the prototype

• Separate connector, not a flag on neon. Picking the connector is the opt-in; it keeps neon-new out of the dependency graph of everyone who just wants to connect to an existing database.
• neon-new is an optional dependency, imported lazily and only when a database actually has to be provisioned, so it stays out of production bundles (CONNECTOR_DEPENDENCIES supports optional: true).
• Refuses to provision when NODE_ENV === "production", with an error pointing at the neon connector.
• Provisioning params (seed, dotEnvFile, dotEnvKey, envPrefix, settings, referrer) are forwarded to instantPostgres and kept out of the ClientConfig passed to Client.

Open questions

• Is a dev-time auto-provisioning connector in scope for db0, or does it belong in a starter/CLI instead?
• neon-new is pre-1.0; is pinning a ^0.x range in the connector metadata acceptable?
• Should the env var it reads (DATABASE_URL) be configurable beyond dotEnvKey?