Use fork digest in gossipsub topic strings

[MegaRedHand]

Summary

• Match the leanSpec gossipsub topic format /leanconsensus/{fork_digest}/{name}/ssz_snappy instead of the custom /leanconsensus/devnet0/... we had hard-coded.

Closes #292.

[greptile-apps]

Greptile Summary

This PR replaces the hardcoded devnet0 string in gossipsub topic strings with a proper ForkDigest([u8; 4]) newtype, matching the leanSpec format /leanconsensus/{fork_digest}/{name}/ssz_snappy. The digest is plumbed through SwarmConfig → BuiltSwarm → P2PServer, with ForkDigest::DUMMY (12345678) as the agreed-upon placeholder until the spec defines real fork identification. The implementation is clean, well-tested, and all existing call sites are correctly updated.

Confidence Score: 5/5

Safe to merge — clean refactor with no logic errors, full test coverage for the new type, and consistent plumbing throughout the stack.

All changes are mechanical substitutions of a hardcoded string with a properly typed value. The ForkDigest Display impl is verified by unit tests, all three topic builders are tested end-to-end, and the runtime message parsing (split("/").nth(3)) is unaffected by the change in position 2 of the topic path. No P0 or P1 findings.

No files require special attention.

Important Files Changed

Filename	Overview
crates/net/p2p/src/gossipsub/messages.rs	Introduces ForkDigest newtype with Display (lowercase hex, no 0x prefix), DUMMY constant, and three topic builder functions; covered by unit tests for all cases.
crates/net/p2p/src/lib.rs	Adds fork_digest field to SwarmConfig, BuiltSwarm, and P2PServer; replaces all inline topic string construction with builder function calls consistently.
crates/net/p2p/src/gossipsub/handler.rs	Updates attestation_subnet_topic fallback call to pass &server.fork_digest; topic parsing via split("/").nth(3) is unaffected and correct for the new format.
bin/ethlambda/src/main.rs	Adds ForkDigest::DUMMY to SwarmConfig with a clear TODO comment explaining when to replace it with a real computed digest.
crates/net/p2p/src/gossipsub/mod.rs	Re-exports updated to expose ForkDigest and topic builder functions instead of the old NETWORK_NAME/BLOCK_TOPIC_KIND/AGGREGATION_TOPIC_KIND constants.
CLAUDE.md	Documentation updated to reflect the new topic format with fork_digest and the 12345678 placeholder, matching the implementation.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["main.rs\nForkDigest::DUMMY"] -->|fork_digest field| B["SwarmConfig"]
    B -->|build_swarm| C["BuiltSwarm\n+ fork_digest"]
    C -->|P2P::spawn| D["P2PServer\n+ fork_digest"]
    B -->|block_topic| E["IdentTopic\nblock"]
    B -->|aggregation_topic| F["IdentTopic\naggregation"]
    B -->|attestation_subnet_topic| G["IdentTopic\nattestation_N"]
    E --> C
    F --> C
    G -->|cached in attestation_topics| C
    D -->|fallback publish| H["attestation_subnet_topic\nfork_digest + subnet_id"]

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