Serve data_column_sidecar events for gossip publication requests

[Bronek]

Add data_column_sidecar to /eth/v1/events. The promise is: these events correspond to what we publish on gossip. An event reports silver's publication request, not successful delivery to a peer.

This PR uses Beacon API v5.0.0-alpha.2 as its specification baseline, rather than released v4.0.0.

This deliberately narrows the alpha's validation event. Upstream covers sidecars received through P2P or API after subnet gossip validation. Silver additionally requires publication, intentionally coupling these notifications to relay policy.

Both publication paths follow silver's gossip checks, including KZG. Gossip columns request SendGossip; RPC columns request PublishDataColumn while synced, regardless of custody. Buffered copies remain silent after their missing block or parent arrives. Syncing RPC, held copies, and EL reconstruction also remain silent. A later acquisition or encoding failure does not retract the original request.

event: data_column_sidecar
data: {"block_root":"0x<64 lowercase hex>","index":"<decimal>","slot":"<decimal>"}

Compatibility: clients requiring v4's kzg_commitments field cannot decode this event. Silver follows the alpha's three-field payload, introduced in v5.0.0-alpha.1. #583 removed kzg_commitments on 2026-03-09 after Gloas removed commitments from sidecars; Fulu sidecars still carry them. Released v4.0.0 retains the field. The inspected go-eth2-client checkout (2026-07-16) and Lighthouse v8.2.2 use the v4 event shape. go-eth2-client rejects an empty commitments array; Lighthouse requires both kzg_commitments and versioned_hashes. Consumers using these decoders, including Xatu and ethereum-metrics-exporter, need v5 support to decode this event.

This PR stacks on bronek/sse_block_gossip_v2, as the second PR above main. It has no prerequisite on PR #231.

• Carry column metadata on publication requests: introduce the metadata enum and enrich both existing requests. Producer tests cover selection, metadata, persistence, and completion; Controller tests verify routing and that conversion adds no second spine publication.
• Serve data_column_sidecar events over SSE: add the topic, renderer, and boundary handling for both request variants. Socket tests cover all three topics, repeats, late subscribers, and engine saturation.
• Clean up temporary test directories and shared memory: give fixtures cleanup owners and unlink Flux objects before removing their link files. This includes pre-existing test leaks found while reviewing the stack.
• Let rust-analyzer see every feature: a root rust-analyzer.toml mirroring just clippy's --all-features, so feature-gated test support stops showing as unresolved imports in editors.

The cleanup commit covers normal teardown and unwinding. Process-global timer objects and cleanup after aborts remain separate work.

Persist, Available, and their consumers remain unchanged. Repeated requests are not deduplicated; late subscribers receive no replay. Additional subscriptions can reach the existing send cap sooner.

The ADR-0004 amendment records the publication contract and deliberate divergence. P2P conformance gaps remain tracked separately.
Message sizes and ring capacities are unchanged.

Coverage limits:

• Synthetic Fulu fixtures pre-seed the signature cache because their validator registry is empty; parent, proposer, inclusion, and KZG checks still run.
• Staged-parent assertions require installed EF fixtures; the test returns early when they are absent.

[vladimir-ea]

what is this change? and given this change doesn't this line above also need to change?

[Bronek]

This is in 0843f87 to stop silver's unit tests leaking temporary files / shmem . I should have created a separate PR for it - sorry about that. It's pure unit tests cleanup.