feat(p2p): log publish-side gossip diagnostics before publish

[MegaRedHand]

What

Emit a diagnostic log line on every gossip publish (block, attestation, aggregation), immediately after snappy encoding and before publish(), so a captured on-wire message can be reconciled against what a peer logs on receipt. Built to debug cross-client snappy/SSZ corruption such as blockblaz/zeam#942.

Fields logged

Field	Meaning
topic	gossipsub topic string (on-wire)
slot	message slot
proposer / block_root	block path only ("if available")
validator	attestation path only
ssz_sha256	SHA256 of the uncompressed SSZ payload
compressed_sha256	SHA256 of the snappy-compressed (on-wire) bytes
compressed_len	on-wire byte length
snappy_self_decode_ok	self round-trip decompress(compressed) == ssz; local-encoder corruption canary
message_id	gossipsub message id, computed via the same function peers use
git_sha	client build SHA
snappy	snappy lib + resolved version (rust-snap/<ver>)

How

• Extracted the message-id logic into a shared gossip_message_id(topic, data); the gossipsub message_id_fn now delegates to it, so the logged id provably matches what peers assign.
• Added crates/net/p2p/build.rs to surface build-time values: VERGEN_GIT_SHA (mirrors the bin crate) and SNAP_VERSION parsed from the workspace Cargo.lock (the snap crate exposes no version const, and Cargo surfaces no dep-version env var).
• Moved hex from a dev-dependency to a regular dependency.

Verification

Ran a local 4-node devnet (20 slots, finalized to slot 19). Confirmed:

• 22 block / 92 attestation / 23 aggregation diagnostic lines, every field populated.
• git_sha=abfdd7b, snappy=rust-snap/1.1.1 (match git rev-parse --short HEAD + Cargo.lock).
• snappy_self_decode_ok=false: 0; receive-side decompress/decode errors: 0.
• Block sizes include the large case (compressed_len=370444, past snappy's 64 KB block boundary) that the #942 corruption requires.

Sample (block):

Publishing block to gossipsub (publish diagnostics) topic=/leanconsensus/12345678/block/ssz_snappy slot=4 proposer=0 block_root=86c842dd…0c98 ssz_sha256=46897d82…02fc compressed_sha256=18008e9e…4b84 compressed_len=370444 snappy_self_decode_ok=true message_id=0fac287f446d6401d67b18119f9f910506f91ce2 git_sha="abfdd7b" snappy="rust-snap/1.1.1"

Notes

• These lines are info-level on every published message, so attestation subnets are chatty. Easy to drop attestation/aggregation to debug and keep blocks at info, or gate behind a flag, if reviewers prefer.