fix(#791): update memory-accounting to reflect eliminated spTxIndex

Remove perSubpathEntryBytes constant and all O(path²) subpath cost
calculations from estimateStoreTxBytes and estimateStoreTxBytesTypical.
Update comments to note that spTxIndex was eliminated in #791 and
singleton subpath entries are dropped from spIndex.

cmd/server/coverage_test.go

@@ -2322,24 +2322,12 @@ func TestSubpathTxIndexPopulated(t *testing.T) {
 	store := NewPacketStore(db, nil)
 	store.Load()
 
-	// spTxIndex must be populated alongside spIndex
-	if len(store.spTxIndex) == 0 {
-		t.Fatal("expected spTxIndex to be populated after Load()")
+	// spIndex must be populated after Load()
+	if len(store.spIndex) == 0 {
+		t.Fatal("expected spIndex to be populated after Load()")
 	}
 
-	// Every key in spIndex must also exist in spTxIndex with matching count
-	for key, count := range store.spIndex {
-		txs, ok := store.spTxIndex[key]
-		if !ok {
-			t.Errorf("spTxIndex missing key %q that exists in spIndex", key)
-			continue
-		}
-		if len(txs) != count {
-			t.Errorf("spTxIndex[%q] has %d txs, spIndex count is %d", key, len(txs), count)
-		}
-	}
-
-	// GetSubpathDetail should return correct match count via indexed lookup
+	// GetSubpathDetail should return correct match count via scan fallback
 	detail := store.GetSubpathDetail([]string{"eeff", "0011"})
 	if detail == nil {
 		t.Fatal("expected non-nil detail for existing subpath")

cmd/server/store.go

@@ -2640,10 +2640,11 @@ func (s *PacketStore) buildDistanceIndex() {
 // usage and independent of GC state.
 //
 // Issue #743: Previous estimates missed major per-packet allocations:
-// - spTxIndex: O(path²) entries per tx (50-150MB at scale)
 // - ResolvedPath on observations (~25MB at scale)
 // - Per-tx maps: obsKeys, observerSet (~11MB at scale)
 // - byPathHop index entries (20-40MB at scale)
+// Note: spTxIndex was eliminated in #791 (saved ~280MB at 3.4M subpaths).
+// Singleton subpath entries are also dropped from spIndex (#791, saves ~150MB).
 const (
 	storeTxBaseBytes  = 384 // StoreTx struct fields + map headers + sync.Once + string headers
 	storeObsBaseBytes = 192 // StoreObs struct fields + string headers
@@ -2657,15 +2658,12 @@ const (
 	// Per path hop: byPathHop index entry (pointer + map bucket)
 	perPathHopBytes = 50
 
-	// Per subpath entry in spTxIndex: string key + slice append + pointer
-	perSubpathEntryBytes = 40
-
 	// Per resolved path element on an observation
 	perResolvedPathElemBytes = 24 // *string pointer + string header + avg pubkey length
 )
 
 // estimateStoreTxBytes returns the estimated memory cost of a StoreTx (excluding observations).
-// Includes per-tx maps (obsKeys, observerSet), byPathHop entries, and spTxIndex subpath entries.
+// Includes per-tx maps (obsKeys, observerSet) and byPathHop entries.
 func estimateStoreTxBytes(tx *StoreTx) int64 {
 	base := int64(storeTxBaseBytes)
 	base += int64(len(tx.RawHex) + len(tx.Hash) + len(tx.DecodedJSON) + len(tx.PathJSON))
@@ -2678,12 +2676,6 @@ func estimateStoreTxBytes(tx *StoreTx) int64 {
 	hops := int64(len(txGetParsedPath(tx)))
 	base += hops * perPathHopBytes
 
-	// spTxIndex: O(path²) subpath combinations
-	if hops > 1 {
-		subpaths := hops * (hops - 1) / 2
-		base += subpaths * perSubpathEntryBytes
-	}
-
 	return base
 }
 
@@ -2697,7 +2689,6 @@ func estimateStoreTxBytesTypical(numObs int) int64 {
 	base += perTxMapsBytes
 	hops := int64(3)
 	base += hops * perPathHopBytes
-	base += (hops * (hops - 1) / 2) * perSubpathEntryBytes
 	// Add observation costs
 	obsBase := int64(storeObsBaseBytes) + 30 + 30 + 60 // observer ID + name + path
 	obsBase += int64(numIndexesPerObs * indexEntryBytes)