v1 packaging: ESM-only, exports map with ./react, no runtime deps, README contract

[dmythro]

Why

The package predates ESM-only tooling; a v1 is the moment to modernise the build and publish surface.

Proposal

• ESM-only, "type": "module", types shipped, exports map with . and ./react, sideEffects: false, no runtime dependencies.
• React as an optional peer (only the ./react entry needs it).
• Size budget in CI (core under a few KB gzipped).
• README: the server-authoritative contract, a minimal example, the React example, a migration note from 0.x.