client: narrow `ReceivedMessage::source` from `SocketAddr` to `SocketAddrV4`

[JustinKovacich]

Context

`src/client/socket_manager.rs:64`:

```rust
/// TODO: narrow `source` to `SocketAddrV4` to match the `TransportSocket`
/// trait's IPv4-only contract — today the field is always a
/// `SocketAddr::V4(_)` wrapping, and the V6 variant is unreachable.
/// Deferred because the rename ripples through `DiscoveryMessage` and
/// `ClientUpdate::Unicast`.
```

Goal

Narrow `ReceivedMessage::source` from `core::net::SocketAddr` to `core::net::SocketAddrV4` so the type-system contract honestly reflects the IPv4-only scope of the `TransportSocket` trait. The V6 arm in pattern-matching call sites is dead today.

Scope

The rename ripples through:

• `ReceivedMessage::source` itself.
• `DiscoveryMessage` (carries the same field type).
• `ClientUpdate::Unicast` (publicly exposes it).
• All pattern-match sites that destructure `SocketAddr::V4(...)` to extract the inner `SocketAddrV4`.

Migration

Breaking change for any downstream code that pattern-matches on `ClientUpdate::Unicast { source: SocketAddr::V4(_) }`. After: `source: SocketAddrV4` directly.