chore: sweep dead helpers across subspecs

[tcoratger]

Summary

Removes top-level functions, methods, and constants that have zero production callers across src/ (verified by grep). All deletions confirmed by grep across src/ and tests/. Tests that exclusively exercised dead code are dropped; tests that incidentally used a removed helper are rewritten against the surviving public API.

This is PR-A in a multi-PR plan from a wider audit of the subspecs codebase.

Diff: 30 files changed, +43 / −1197.

Removed surface

• ENR: has, is_compatible_with
• Eth2Data: no_scheduled_fork
• Multihash: identity, sha256 (logic inlined into from_data)
• PeerId: from_secp256k1 (callers go through IdentityKeypair.to_peer_id / PeerId.from_public_key)
• Secp256k1PublicKey: from_bytes
• IdentityKeypair: from_bytes, private_key_bytes
• MessageCache, SeenCache: clear, __len__
• RPC: is_empty (ControlMessage.is_empty stays — it is used by RPC.encode)
• GossipsubBehavior: set_message_handler, _message_handler field, the dispatch branch in _handle_message
• Fp: two_adic_generator, plus TWO_ADIC_GENERATORS and TWO_ADICITY constants
• SyncService: process_pending_blocks, start_sync, _sync_lock
• HeadSync: process_all_processable
• PeerManager: clear
• BlockCache: get_orphan_parents
• ValidatorRegistry: from_secret_keys
• GenesisConfig: create_state

Deliberately kept (against the audit)

• decode_request in reqresp/codec.py — the audit flagged it as dead because the inbound handler reimplements the logic inline, but it has rich wire-format test coverage and is the symmetric counterpart of encode_request. The cleaner fix is to make the handler use decode_request (separate PR).
• ENR.udp6_port, quic6_port, attestation_subnets, sync_committee_subnets, is_aggregator + the matching ENR keys (UDP6, QUIC6, ATTNETS, SYNCNETS, IS_AGGREGATOR) — used by packages/testing/src/consensus_testing/test_fixtures/networking_codec.py to build cross-client spec fixtures. Not visible from a src/-only grep.
• GossipsubMessage.snappy_decompress field — borderline; reflects libp2p protocol behavior the spec should preserve.

Out of scope (deferred)

• The whole subspecs/networking/discovery/ package (PR-B decision pending: wire in or move to experimental/).
• Database Protocol slim-down + namespace dataclass collapse (PR-I).
• SyncService.reset / BackfillSync.reset / HeadSync.reset (deferred to PR-J — likely a deliberate test-cleanup API).

Test plan

• uvx tox -e all-checks passes (ruff, format, ty, codespell, mdformat).
• Reviewer to spot-check: any of the removed helpers still wanted as part of the public API?
• Reviewer to spot-check: each deleted test was either exclusively for a removed helper, or rewritten to exercise the surviving API.

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