feat(p2p): add opt-in discv5 peer discovery - #579

Lean nodes could only meet through a static bootnode list, so every new
node needed an operator to hand it peers. This wires ethrex's discv5
stack in behind `--discovery.enable`: the node builds and signs its own
ENR, joins the DHT on its own UDP socket, and dials what it finds over
libp2p QUIC. Static bootnode dialing is untouched and discovery is off by
default, so nothing changes for an operator who does not ask for it.

Admission follows the beacon phase0 p2p spec, mirroring lighthouse's
`eth2_fork_predicate`: the `eth2` fork digest must match, a differing
`next_fork_version`/`next_fork_epoch` is explicitly tolerated, and the
peer must advertise a `quic` port. The checks live in a `LeanFilter` that
ethrex's peer table runs as each ENR arrives, so a record is judged where
it lands rather than at dial time, and is judged afresh whenever the peer
publishes a higher-`seq` record. Survivors are ranked by how many
attestation subnets they cover that no connected peer does, so discovery
fills subnet gaps first. A peer's `attnets` is self-reported, so subnet
ids at or beyond the local committee count are dropped before ranking
sees them.

`ethrex-p2p` is pinned to the unmerged `feat/discovery-peer-requirements`
branch, which carries the unified `DiscoveryServer`, the peer table, and
the `PeerFilter` seam. Repoint it at a main revision once that merges.

Known gap: `DiscoveryServer::spawn` builds its own local record and
offers no way to seed the consensus entries, so the ENR ethrex answers
queries with carries `ip`/`udp`/`secp256k1` but not `eth2`, `attnets` or
`quic`. Discovery is one-sided until `spawn` can take a prepared record:
we find and admit lean peers, but a lean peer applying these same rules
to what ethrex serves would refuse us. See `docs/discovery.md`.

Quality pass over the discovery feature. No behaviour change; the one
observable difference is that a malformed bootnode ENR now warns once
instead of twice, because the file is parsed once.

Reuse and layering:

- Merge `ethlambda-types::enr` into `p2p::discovery::enr`. The shared
  types crate grew an SSZ container and a `libssz_derive` use for a
  single consumer crate, and split `encode_attnets` from the
  `ATTNETS_ENR_KEY` that gives it meaning. `FORK_DIGEST` stays in
  `types::constants`, where a second crate does use it.
- Move the dial loop out of `lib.rs` into `discovery::dial`, matching how
  `gossipsub::handler` and `req_resp::handlers` already keep their bodies
  out of the shared actor file. `DiscoveryState`, `covered_subnets` and
  `local_peer_id` go with it, so dial policy is editable without touching
  shared actor state.
- Add `P2PServer::forget_discovered_peer` so the two teardown paths
  (`ConnectionClosed` and `OutgoingConnectionError`) share a seam instead
  of both reaching into `peer_attnets`.
- One `quic_multiaddr()` for the two dial paths that were building the
  same `ip / udp / quic-v1 / p2p` chain, and
  `ethrex_p2p::utils::public_key_from_signing_key` in place of the
  hand-rolled uncompressed-SEC1 conversion (which had three copies).
- Fold the `!= 0` filter into `read_quic_port` and have `parse_enr` call
  it. The two spellings of "no dialable quic port" had already drifted.
- Drop `read_extra`: ethrex's `pairs.extra()` already returns `Bytes`, so
  the wrapper only added a copy on a path that runs per arriving ENR.

Simplification:

- `subnets_from_attnets(bits, committee_count)` replaces
  decode-everything-then-clamp. Iterating our own committee makes the
  clamp unforgettable rather than documented in three places, and stops
  a padded hostile bitfield allocating ~18 KB before being discarded.
- `DiscoveryError` via `thiserror` replaces 12 hand-rolled `String`
  errors; p2p was the only crate in the workspace without it. `main.rs`
  loses its `map_err(|err| eyre::eyre!(err))` bridge.
- Delete `DiscoveredPeer::label` and `DiscoveryHandle::bound_addr`: both
  were read only by tests, and `label` allocated a base58 string on every
  admission while the one log line uses `%peer_id`. The ENR-vs-bound-port
  test now asserts on the record's `udp` entry, which is the invariant.
- Delete `RejectReason::as_str`, whose five strings restated the five
  variant docs for one `debug!`.
- Read the bootnode file once (`#[derive(Clone)] Bootnode`) and inline
  the locals copied out of `options.discovery`. Move the unspecified-IP
  warning into `spawn_discovery`, next to the code that picks the value.

Dependencies:

- `DiscoverySpawnConfig::node_key` takes `Vec<u8>` like its sibling
  `SwarmConfig::node_key`, which removes the binary's direct `secp256k1`
  dependency and its version-coupling to ethrex's workspace.
- p2p: drop the unused `recovery` feature, move `bytes` and `rand` to
  dev-dependencies (both are test-only).

Revert `pub mod req_resp` / `pub mod encoding` to private: they were
widened for an `examples/mainnet_gossip.rs` that is not in the tree, and
making `req_resp` public also exposed the actor-facing `handlers` module.

`NodeIdentity` reaches the identity route behind an `Arc`, so a polled
endpoint stops cloning two startup-fixed strings per request.