combined adjustable + mergeable submissions

[ckoopmann]

Issue

A submission can carry bid adjustment data or block merging data, but not both. The two are signaled independently (WITH_ADJUSTMENTS flag, merge_type), but no body encoding carries both extensions, and the decoder drops one of the two when both are signaled: the dehydrated mergeable path ignores the with-adjustments flag, and the with-adjustments path errors on merge_type: mergeable.

A builder submitting mergeable blocks therefore loses bid adjustment on those bids entirely.

Solution

Add a combined body encoding — core fields ++ bid_adjustment_data ++ merging_data — selected by the existing signals (with-adjustments flag set and merge_type: mergeable), and have the decoder return both extensions. Existing single-extension wire formats are unchanged.

The body layout matches the combined submission format already accepted by the ultra sound relay, so builders can use a single encoder for both relays.

Details

• new flat body types SignedBidSubmissionWithAdjustmentsAndMergingData and DehydratedBidSubmissionFuluWithAdjustmentsAndMergingData carrying both bid_adjustment_data and merging_data (core fields ++ bid_adjustment_data ++ merging_data)
• decoder dispatches on (with_adjustments, merge_type) instead of treating them as exclusive: dehydrated and default paths decode the combined body when the with-adjustments flag is set and merge_type is mergeable, returning both merging and adjustment data
• existing single-extension wire formats and AppendOnly / mark_all_txs_mergeable behavior unchanged
• tests: SSZ round-trips for both new types, flat byte-layout assertion, decoder dispatch tests for both paths