evn flag to: don't merge Hist/II but still merge Domains (#19438)

Run with `NO_MERGE_HISTORY=true`: `11.1blk/sec -> 14.3blk/sec` (measured
at 12M block range)
`NO_MERGE_HISTORY=1 COMPRESS_WORKERS=15 MERGE_WORKERS=1
COLLATE_WORKERS=4`

Reason is unclear: 
- maybe it's because of less domain files
- maybe it's because of less impact of merge to exec
will do one more experiment to understand:
#19441

<img width="364" height="886" alt="Screenshot 2026-02-24 at 14 31 52"
src="https://github.com/user-attachments/assets/3180c708-8851-4a48-a6a7-78ca54bd552c"
/>

Author: AskAlexSharov

common/dbg/experiments.go

@@ -56,7 +56,8 @@ var (
 
 	//state v3
 	noPrune              = EnvBool("NO_PRUNE", false)
-	noMerge              = EnvBool("NO_MERGE", false)
+	noMerge              = EnvBool("NO_MERGE", false)         // don't merge Domain/Hist/II
+	noMergeHistory       = EnvBool("NO_MERGE_HISTORY", false) // don't merge Hist/II but still merge Domain
 	discardCommitment    = EnvBool("DISCARD_COMMITMENT", false)
 	pruneTotalDifficulty = EnvBool("PRUNE_TOTAL_DIFFICULTY", true)
 
@@ -120,6 +121,7 @@ func ReadMemStats(m *runtime.MemStats) {
 func DiscardCommitment() bool    { return discardCommitment }
 func NoPrune() bool              { return noPrune }
 func NoMerge() bool              { return noMerge }
+func NoMergeHistory() bool       { return noMergeHistory }
 func PruneTotalDifficulty() bool { return pruneTotalDifficulty }
 
 var (

db/state/merge.go

@@ -147,6 +147,10 @@ func (dt *DomainRoTx) findMergeRange(maxEndTxNum, maxSpan uint64) DomainRanges {
 
 func (ht *HistoryRoTx) findMergeRange(maxEndTxNum, maxSpan uint64) HistoryRanges {
 	var r HistoryRanges
+	if dbg.NoMergeHistory() {
+		return r
+	}
+
 	mr := ht.iit.findMergeRange(maxEndTxNum, maxSpan)
 	r.index = *mr