wip:feat: update chain on fcu unwind

[mattsse]

ref #17798

just a smol comment re what would need to be done here in order to support this usecase that isn't allowed by the ethereum spec, but I'm unsure if we should even support unwinding the chain like this.
the intended use case for this was originally so that op can build an alternative chain based on a canonical ancestor block. the limitation with how we currently handle this is that this doesn't reset the node's LATEST state so until the new alternative block is applied the current head block remains the LATEST.

we effectively would need to do step 3 here as well, but this becomes slightly more difficult if a child block of the new head is already committed to disk

reth/crates/engine/tree/src/tree/mod.rs

Lines 876 to 893 in f3a3fff

	// 3. ensure we can apply a new chain update for the head block
	if let Some(chain_update) = self.on_new_head(state.head_block_hash)? {
	let tip = chain_update.tip().clone_sealed_header();
	self.on_canonical_chain_update(chain_update);
	// update the safe and finalized blocks and ensure their values are valid
	if let Err(outcome) = self.ensure_consistent_forkchoice_state(state) {
	// safe or finalized hashes are invalid
	return Ok(TreeOutcome::new(outcome))
	}
	if let Some(attr) = attrs {
	let updated = self.process_payload_attributes(attr, &tip, state, version);
	return Ok(TreeOutcome::new(updated))
	}
	return Ok(valid_outcome(state.head_block_hash))
	}

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[codspeed-hq]

CodSpeed Performance Report

Merging #17857 will improve performances by 10.99%

_{Comparing matt/add-note-about-fcu-unwind (f3a3fff) with main (ee8c893)}

Summary

⚡ 1 improvements
✅ 76 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	hash builder[init size 10000 | update size 100 | num updates 1]	9.6 ms	8.7 ms	+10.99%