Build a block for each slot from the node's mempool

[0w3n-d]

Issue: #550 (step 3 of 6)

What this PR does

Builds a block for each slot the watcher publishes. The role now runs end to
end up to the point of submission: it fills from the node's mempool, pays the
proposer, and logs the block and its value.

create_payload(gas_ceil = registered_gas_limit)   // ethrex applies the 1/1024 clamp
PayloadBuildContext::new -> apply_system_operations
ctx.remaining_gas -= payout_gas_reserve           // reserve
fill_transactions                                  // tip-sorted, from the mempool
ctx.remaining_gas += payout_gas_reserve            // restore
apply_tx_to_payload(payout)                        // trailing transfer
extract_requests -> apply_withdrawals -> finalize_payload

The coinbase is the builder, so tips accrue here and the bid is

payout = ctx.block_value + subsidy_wei - payout_gas_reserve * base_fee

Reserving the full payout_gas_reserve rather than the actual 21000 is
deliberate: it under-credits the builder slightly and can never overdraw it.

fill_transactions gates only on ctx.remaining_gas, which is pub, so the
reservation needs no ethrex change.

Two fixes to earlier steps

• The default subsidy was unusable. At 1 gwei it could not cover the
  payout's own gas (21000 x base fee, around 2e13 wei), so every build with an
  idle mempool failed. Raised to 0.001 ETH. an_empty_mempool_still_bids_the_subsidy
  caught this; step 1's assertions are updated to match.
• The step 2 SlotContext drain in main.rs is replaced by the real consumer,
  so nothing is dead.

What this PR deliberately does not do

No signing and no submission — BuiltBlock's blobs_bundle, requests and
account_updates carry #[allow(dead_code)] until step 4 consumes them. No
custom ordering: this uses ethrex's tip-sorted fill_transactions. No in-slot
rebuilding; the block is built once per slot event.

Tests

13 new, written before the implementation and signed off first, all against a
real chain (dev_genesis_store + funded_signers) with transactions admitted
through add_transaction_to_pool.

• The block (3): mempool txs are included; the block extends the parent with
  the builder as coinbase; the header gas limit follows the registered limit
  through ethrex's clamp.
• The payout (4): it is last, is a plain zero-tip transfer to the fee
  recipient, and pays exactly the bid; the bid matches the formula; the fee
  recipient's balance rises by the bid, checked against the parent state the
  way the relay's paid_by_balance does; and the reserve survives a full block
  — without it that test fails, which is the point.
• Refusals (4): an idle mempool still bids the subsidy; zero subsidy with no
  tips is refused; an unaffordable payout is refused; paying ourselves is
  refused. A bad slot is skipped with a typed error, never submitted broken.
• Consensus (2): withdrawals are applied; a blob transaction reaches the block
  with the sidecar the mempool holds.

127 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