Merge beacon payload attributes with relay duties into a slot context

[0w3n-d]

Issue: #550 (step 2 of 6)

What this PR does

Adds SlotTracker, which merges the beacon node's payload_attributes SSE
events with the relay's proposer duties and emits one SlotContext per slot
worth building. Neither source alone is enough: the beacon node gives the
parent, timestamp, prev_randao, withdrawals and parent_beacon_block_root,
and the relay gives the proposer pubkey, fee recipient and registered gas
limit.

The building role now runs. It subscribes, polls duties every 12s, and logs
each slot it would build for. Nothing is built or submitted yet.

The fee recipient and gas limit are taken from the duty, never from the
event's suggested_fee_recipient — that field is the local validator's, and
paying it would produce a block the relay rejects.

What this PR deliberately does not do

No block assembly, no submission. The SlotContext receiver in main.rs is a
drain until step 3. SlotContext carries #[allow(dead_code)] until then.

Duties are deserialized into the relay's own BuilderGetValidatorsResponse
rather than a local struct, which keeps the two in lockstep but means this
builder targets Helix, not any mev-boost relay.

Tests

11 new, written before the implementation and signed off first. The merge
logic is pure and carries all of them; the SSE and HTTP calls stay thin and
are exercised by running the role.

• Merging (3): an event plus a matching duty yields a context; the fee
  recipient and gas limit come from the duty, not the event; an event with no
  duty is skipped.
• Which events to act on (4): a repeated event is skipped, since the beacon
  node re-emits whenever it recomputes; a new parent for the same slot is
  not a duplicate and must rebuild, because the relay keys attributes by
  (parent_hash, slot); an older slot is discarded, as an SSE reconnect
  replays; a missing parent_beacon_block_root skips the slot, since EIP-4788
  makes it mandatory.
• Duty cache (2): a refresh replaces the previous set; past slots are pruned so
  duties do not accumulate across epochs.
• Wire contract (2): fixtures for a real get_validators response and a real
  beacon SSE payload.

The get_validators fixture already earned its place — it caught that
Filtering serializes as "global", not 0.

114 pass in the crate.

Reviewer checklist

• CI (lint, unit-test) is green
• Matches the linked issue/step
• No unexplained scope creep or unrelated files touched