sigilry/sigilry: packages/dapp/discovery.spec.md

# Provider Discovery & Push-Event Channel — `@sigilry/dapp` Spec Supporting spec for [`./SPEC.md`](./SPEC.md). Covers the two capabilities `@sigilry/dapp` is missing to be a complete, announce-first Canton dApp toolkit: **provider discovery** (the `canton:announceProvider` tier) and the **push-event