misc code improvements tracked from TODO cleanup

[ethvibes]

Description

Remaining actionable TODOs from the #31 cleanup, grouped by area.

Boot node — DONE

• Add DOS prevention CLI params — removed (boot node is discv5-only, inapplicable)
• Generalise boot node config to multiaddr — removed (same reason)

EIP-7892 blob limits — BLOCKED (external spec)

• Remove max_blobs_per_block_within_fork workaround once fork-version changes on BPO forks (consensus/types/src/chain_spec.rs:873)
• Decide default for blob params schedule (consensus/types/src/chain_spec.rs:1802)
• Fix blob count overestimation in API (common/eth2/src/lib.rs:1523)

Crypto — BLOCKED (blst upstream)

• Remove unsafe blst code block once safe API is provided (crypto/bls/src/impls/blst.rs:62)

Execution layer — DONE

• Add EL requests in mock EL — DONE: From<ExecutionRequests<E>> for JsonExecutionRequests, configurable mock EL context, wired into V4/V5 handlers
• Refactor error enums (beacon_node/execution_layer/src/lib.rs:194,217) — non-critical code quality, leaving as-is

Beacon chain

• Persist aggregation/sync contribution pools to disk (beacon_node/beacon_chain/src/builder.rs:1064) — non-critical feature
• Remove PeerDAS checkpoint sync condition in store tests — BLOCKED (PeerDAS checkpoint sync not yet implemented)

Network — DONE

• Refactor DataColumnsByRootRequestId — DONE
• Shift subnet service to dynamic bitfield — removed (HashSet is fine for ≤64 subnets)

HTTP API — DONE

• Make attestation test fork-agnostic — DONE

Operation pool — DONE

• Lock optimization — DONE: pre-electra forks use read lock instead of write-then-downgrade

Store — DONE

• Dynamic buffer allocation for hdiff — DONE

Test utils — DONE

• Fix non-deterministic SecretKey/Signature generation — DONE

Summary

Done: 10 items (boot node ×2, network ×2, HTTP API, EL mock, op pool, store, test utils, slasher diagnostics)
Blocked: 5 items (EIP-7892 ×3, blst safe API, PeerDAS checkpoint sync)
Remaining (non-critical): 2 items (EL error enum refactor, pool disk persistence)