feat: Gloas fork choice with payload status

[Savid]

Gloas fork choice in forky: payload-status-aware nodes from beacon nodes and Xatu, shown in the graph.

Changes

• Fork choice with payload status. Forky's own fork choice type (internal/types) replaces go-eth2-client's v1 type in frames. Post-Gloas a block can have a pending, an empty and a full node, each with its parent payload status, checkpoint epochs and PTC counts. The store's extra_data is kept.
• Beacon node source fetches GET /eth/v2/debug/fork_choice via go-eth2-client v0.1.10, which decodes it strictly to ethereum/beacon-APIs#615 as merged. Nodes without the endpoint, or with a non-spec response (ErrInvalidResponse, i.e. every client today), use v1 for an hour before v2 is retried; other failures fall back for that fetch only.
• Xatu source decodes events with DiscardUnknown, so new Xatu fields don't drop batches. Events with payload statuses (from sentries running feat(sentry): fetch Gloas-aware v2 debug fork choice xatu#900) keep their empty and full nodes; events without (every sentry today) are normalised as v1.
• Normalisation. v1 dumps that return several nodes per Gloas block are collapsed to one per block. v2 nodes have empty/full parents mapped back to the parent block's root, and any values a node lacks are filled in from its extra_data or its block's other nodes. Lighthouse's not_yet_revealed validity is shown as optimistic.
• Web: graph nodes show empty and full payloads, a legend, rich hover cards and a payload "tide" view.
• go-eth2-client moves from attestantio to ethpandaops (v0.1.10); xatu bumped to the feat(sentry): fetch Gloas-aware v2 debug fork choice xatu#900 commit.

Compatibility

• Works with the Xatu currently in production: events from today's sentries (no payload status) were captured live from Sepolia (Teku, Prysm, Nimbus, Lighthouse) and mainnet (Prysm, Lighthouse) and all decode through this branch's path.
• Frames stored by 1.0.2 still load (TestFrameLegacyJSON). No database migrations.
• Deploy this before feat(sentry): fetch Gloas-aware v2 debug fork choice xatu#900. Forky 1.0.2 decodes strictly and would drop batches containing #900's new fields.

Verified

• go test ./internal/... and golangci-lint pass.
• Live against Sepolia: a Teku beacon node source backs off to v1 (71 nodes, one per block); the same node behind a proxy that reshapes its v2 response to the merged spec goes through v2 (213 nodes: pending/empty/full for 71 blocks, all with payload status, PTC counts and epochs, parent links correct).

Follow-up

Once ethpandaops/xatu#900 is merged and released, bump xatu from the PR commit to the release tag.

🤖 Generated with Claude Code

[redpandabot]

This PR adds Gloas payload-status-aware fork choice: a forky-owned ForkChoice/ForkChoiceNode type replacing go-eth2-client's v1 type, a v2 beacon-node source with an hour-long v1 back-off for unsupported/non-spec responses, Xatu decoding that preserves payload nodes, normalization across v1/v2/partial clients, and graph/tide UI. I traced the normalization, enum mappings (including xatu's 0=EMPTY/1=FULL/2=PENDING against the pinned commit), API schema, and frame round-trip and found no confirmed blocker or concern.

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_{Reviewed 49 changed file(s) @ 8ec8ec90 — no blocking issues found.
"The bug is always in the last place you look — because you stop looking after you find it."}