refactor(networking): remove discv5 package and test surface

[tcoratger]

Summary

• Deletes the ~3,300 LOC subspecs/networking/discovery/ package along with its unit tests, consensus spec fillers, and the associated test-fixture infrastructure (~14k lines removed net).
• Keeps subspecs/networking/enr/ — the ENR record format is shared by every lean client.
• Updates two stale docstrings in peer.py / types.py that previously referred to data being "populated from discovery".

Why

Cross-client survey of production lean Ethereum clients:

Client	Uses discv5?	What it uses
Ream (lean target)	No	libp2p + identify + req_resp + gossipsub; static ENR/multiaddr bootnodes from config
ethlambda	No	libp2p + QUIC; parse_enrs() from a bootnodes: Vec<Bootnode> config for static dialing
zeam	No	libp2p (eth-p2p-z) + QUIC; ENR-from-yaml for static peers
Lighthouse (beacon, reference)	Yes	Full discv5 — but lean clients are explicitly not replicating this

Notably, Ream's repo contains a discv5 Rust crate but its lean ReamBehaviour composes only identify / req_resp / gossipsub / slow_peer_banlist / connection_limits — no discovery behaviour. The discv5 crate is used only by Ream's beacon-chain target.

The discv5 wire protocol implementation in leanSpec appears to have been carried over from beacon-chain assumptions and has zero production consumers across the lean ecosystem.

Scope of deletion

• src/lean_spec/subspecs/networking/discovery/ — 11 production files
• tests/lean_spec/subspecs/networking/discovery/ — 15 unit test files
• tests/consensus/lstar/networking/test_discv5_*.py and test_discovery_*.py — 5 consensus spec fillers (no lean client consumes the JSON vectors they emitted)
• packages/testing/src/consensus_testing/test_fixtures/discovery_crypto.py (fixture class)
• discv5 sections of packages/testing/src/consensus_testing/test_fixtures/networking_codec.py (dispatch cases, decoder map entries, _make_discv5_message, _make_discv5_packet, _make_xor_distance, _make_log2_distance, _build_discv5_message)
• make_challenge_data helper in tests/lean_spec/helpers/builders.py (used only by deleted unit tests)
• Public exports of DiscoveryCryptoTest / DiscoveryCryptoTestFiller

Test plan

• ruff check — passes
• ruff format --check — passes
• ty check — passes
• codespell — passes
• mdformat --check docs/ — passes
• uv lock --check — passes
• Full pytest suite on CI

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