refactor: move block merging logic into simulator

[tosynthegeek]

Move block merging from auctioneer to simulator tile

Closes #438

Summary

Eliminates the merge round-trip through the auctioneer. Previously, the auctioneer tracked merge state, sent SimRequest::Merge to the simulator, received the response as Event::MergeResult, and assembled the merged payload. Now the simulator tile owns the entire merge lifecycle and delivers a finished PayloadEntry directly to the auctioneer.

Changes

spine/

• messages.rs: Added TopBidForSim (auctioneer → sim tile, base block tracking) and MergedBlockSubmission (sim tile → auctioneer, merged payload ready). Added NO_IL_IX sentinel for slots without an inclusion list.
• mod.rs: Added top_bid_for_sim and merged_block queues to HelixSpine.

simulator/

• block_merger.rs (new): SimBlockMerger owns order pool, appendable block tracking, merge request building, and payload assembly (assemble_payload). Takes RelayConfig so inclusion_list config is accessible. take_base_info provides blobs bundle and builder pubkey for assembly. add_inclusion_list correctly reads from relay_config.inclusion_list.
• mod.rs: Removed SimRequest::Merge. SimResult replaces Merge with MergedPayload(Box<PayloadEntry>).
• tile.rs: SimulatorTile now has SimBlockMerger, sim_il, merging_enabled. After validation succeeds, ValidatedMergeCandidate is built from pre-captured fields (before the async spawn) and sent back via ValidationDone. handle_merge_done assembles the payload synchronously on the tile thread. on_new_slot seeds the IL from sim_il using NO_IL_IX sentinel. Consumes TopBidForSim to update the base block.

auctioneer/

• merged_bid_cache.rs (renamed from block_merger.rs): Stripped to MergedBidCache for get_header tracking plus get_mergeable_orders. All order pooling and request building removed.
• context.rs: BlockMerger removed from SlotContext, replaced by MergedBidCache. Added sim_il: Arc<SharedVector<InclusionListWithMetadata>>. on_new_slot pushes IL into sim_il and passes the index in ToSimMsg::ix. handle_merged_payload replaces handle_merge_response.
• submit_block.rs: MergeData construction removed. Produces TopBidForSim when is_top_bid is true (both optimistic and post-simulation paths). block_merger.update_base_block calls removed.
• mod.rs: Event::MergeResult replaced by Event::MergedPayload(Box<PayloadEntry>). process_slot_data no longer calls block_merger.add_inclusion_list. Three on_new_slot call sites pass il.clone(). MergedBlockSubmission consumed separately from FromSimMsg.

main.rs

• sim_il shared vector created and passed to both SimulatorTile::create and Auctioneer::new.