webrtc: react-native-webrtc as a regular dependency pulls react-native and metro into every Node.js install

Nobody has claimed this yet.

### Description

`@libp2p/webrtc` declares `react-native-webrtc` in `dependencies` (range `^124.0.6` as of 6.0.24; our lockfile resolves 124.0.7). `react-native-webrtc` declares `react-native >=0.60.0` as a peer dependency, and npm and pnpm auto-install non-optional peers, so Node.js consumers install `react-native` and its `metro` toolchain. That includes projects that reach `@libp2p/webrtc` transitively through `helia` or `@helia/verified-fetch` and never import `react-native-webrtc`.

The runtime is already platform-split: Node resolves `dist/src/webrtc/index.js`, which uses `node-datachannel/polyfill`, and only `dist/src/webrtc/index.react-native.js` imports `react-native-webrtc`, selected through the existing `react-native` field in `package.json`. The install graph is what does not match the runtime graph.

### Impact

The subtree is large (react-native 0.85.3 plus metro 0.84.4 and their dependents), and metro pins `image-size` 1.x, which carries two advisories with no patched release (GHSA-5p2g-fcmc-qvqq, GHSA-w3rx-r6r6-pgpr). For Node consumers this is permanent audit noise from an unused install subtree, not a runtime vulnerability, on top of the install weight.

Reproduce from any Node project that depends on `helia` or `@helia/verified-fetch` (observed with pnpm 10; npm's peer auto-install behaves the same):

```bash
pnpm why react-native
pnpm why image-size
```

### Proposed next step

Move `react-native-webrtc` from `dependencies` to an optional peer:

```json
"peerDependencies": {
"react-native-webrtc": "^124.0.6"
},
"peerDependenciesMeta": {
"react-native-webrtc": {
"optional": true
}
}
```

React Native consumers install it explicitly alongside `@libp2p/webrtc`; Node consumers skip the subtree. `optionalDependencies` would not help here, since package managers still install those by default. Because the `react-native` field already isolates the import, no code change looks necessary beyond possibly a clearer error when the React Native entry runs without the peer installed.

Start with @libp2p/webrtc's package.json, then inspect its existing node and react-native entry points. Verify the dependency graph with pnpm why react-native and pnpm why image-size; done means Node consumers no longer install the React Native subtree while the React Native entry still resolves its peer explicitly.

Written by the indexing model from the issue text.