chore(db): adjust db bench name to improve benchstat grouping (#10283)

The benchstat tool allows custom grouping when comparing with what it
calls "sub-name configuration keys":

https://pkg.go.dev/golang.org/x/[email protected]/cmd/benchstat#hdr-Configuring_comparisons

That's quite useful for these benchmarks, as we basically have two
independent configs: The type of benchmark and the size. Real example
usage for the prepared named statements PR (results are rubbish for
unrelated reasons):

```
$ benchstat -row ".name /n" bench-main.out bench-prepared.out
goos: linux
goarch: amd64
pkg: github.com/syncthing/syncthing/internal/db/sqlite
cpu: Intel(R) Core(TM) i5-8250U CPU @ 1.60GHz
                            │ bench-main-20250823_014059.out │   bench-prepared-20250823_022849.out   │
                            │             sec/op             │     sec/op      vs base                │
Update Insert100Loc                           248.5m ±  8% ¹   157.7m ±  7% ¹  -36.54% (p=0.000 n=50)
Update RepBlocks100                           253.7m ±  4% ¹   163.6m ±  7% ¹  -35.49% (p=0.000 n=50)
Update RepSame100                            130.42m ±  3% ¹   60.26m ±  2% ¹  -53.80% (p=0.000 n=50)
Update Insert100Rem                           38.54m ±  5% ¹   21.94m ±  1% ¹  -43.07% (p=0.000 n=50)
Update GetGlobal100                          10.897m ±  4% ¹   4.231m ±  1% ¹  -61.17% (p=0.000 n=50)
Update LocalSequenced                         7.560m ±  5% ¹   3.124m ±  2% ¹  -58.68% (p=0.000 n=50)
Update GetDeviceSequenceLoc                  17.554µ ±  6% ¹   8.400µ ±  1% ¹  -52.15% (n=50)
Update GetDeviceSequenceRem                  17.727µ ±  4% ¹   8.237µ ±  2% ¹  -53.54% (p=0.000 n=50)
Update RemoteNeed                              4.147 ± 77% ¹    1.903 ± 78% ¹  -54.11% (p=0.000 n=50)
Update LocalNeed100Largest                   21.516m ± 22% ¹   9.312m ± 47% ¹  -56.72% (p=0.000 n=50)
geomean                                       15.35m           7.486m          -51.22%
¹ benchmarks vary in .fullname
```

internal/db/sqlite/db_bench_test.go

@@ -60,7 +60,7 @@ func BenchmarkUpdate(b *testing.B) {
 			}
 		}
 
-		b.Run(fmt.Sprintf("Insert100Loc@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=Insert100Loc/size=%d", size), func(b *testing.B) {
 			for range b.N {
 				for i := range fs {
 					fs[i] = genFile(rand.String(24), 64, 0)
@@ -72,7 +72,7 @@ func BenchmarkUpdate(b *testing.B) {
 			b.ReportMetric(float64(b.N)*100.0/b.Elapsed().Seconds(), "files/s")
 		})
 
-		b.Run(fmt.Sprintf("RepBlocks100@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=RepBlocks100/size=%d", size), func(b *testing.B) {
 			for range b.N {
 				for i := range fs {
 					fs[i].Blocks = genBlocks(fs[i].Name, seed, 64)
@@ -86,7 +86,7 @@ func BenchmarkUpdate(b *testing.B) {
 			b.ReportMetric(float64(b.N)*100.0/b.Elapsed().Seconds(), "files/s")
 		})
 
-		b.Run(fmt.Sprintf("RepSame100@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=RepSame100/size=%d", size), func(b *testing.B) {
 			for range b.N {
 				for i := range fs {
 					fs[i].Version = fs[i].Version.Update(42)
@@ -98,7 +98,7 @@ func BenchmarkUpdate(b *testing.B) {
 			b.ReportMetric(float64(b.N)*100.0/b.Elapsed().Seconds(), "files/s")
 		})
 
-		b.Run(fmt.Sprintf("Insert100Rem@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=Insert100Rem/size=%d", size), func(b *testing.B) {
 			for range b.N {
 				for i := range fs {
 					fs[i].Blocks = genBlocks(fs[i].Name, seed, 64)
@@ -112,7 +112,7 @@ func BenchmarkUpdate(b *testing.B) {
 			b.ReportMetric(float64(b.N)*100.0/b.Elapsed().Seconds(), "files/s")
 		})
 
-		b.Run(fmt.Sprintf("GetGlobal100@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=GetGlobal100/size=%d", size), func(b *testing.B) {
 			for range b.N {
 				for i := range fs {
 					_, ok, err := db.GetGlobalFile(folderID, fs[i].Name)
@@ -127,7 +127,7 @@ func BenchmarkUpdate(b *testing.B) {
 			b.ReportMetric(float64(b.N)*100.0/b.Elapsed().Seconds(), "files/s")
 		})
 
-		b.Run(fmt.Sprintf("LocalSequenced@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=LocalSequenced/size=%d", size), func(b *testing.B) {
 			count := 0
 			for range b.N {
 				cur, err := db.GetDeviceSequence(folderID, protocol.LocalDeviceID)
@@ -146,7 +146,7 @@ func BenchmarkUpdate(b *testing.B) {
 			b.ReportMetric(float64(count)/b.Elapsed().Seconds(), "files/s")
 		})
 
-		b.Run(fmt.Sprintf("GetDeviceSequenceLoc@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=GetDeviceSequenceLoc/size=%d", size), func(b *testing.B) {
 			for range b.N {
 				_, err := db.GetDeviceSequence(folderID, protocol.LocalDeviceID)
 				if err != nil {
@@ -154,7 +154,7 @@ func BenchmarkUpdate(b *testing.B) {
 				}
 			}
 		})
-		b.Run(fmt.Sprintf("GetDeviceSequenceRem@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=GetDeviceSequenceRem/size=%d", size), func(b *testing.B) {
 			for range b.N {
 				_, err := db.GetDeviceSequence(folderID, protocol.DeviceID{42})
 				if err != nil {
@@ -163,7 +163,7 @@ func BenchmarkUpdate(b *testing.B) {
 			}
 		})
 
-		b.Run(fmt.Sprintf("RemoteNeed@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=RemoteNeed/size=%d", size), func(b *testing.B) {
 			count := 0
 			for range b.N {
 				it, errFn := db.AllNeededGlobalFiles(folderID, protocol.DeviceID{42}, config.PullOrderAlphabetic, 0, 0)
@@ -178,7 +178,7 @@ func BenchmarkUpdate(b *testing.B) {
 			b.ReportMetric(float64(count)/b.Elapsed().Seconds(), "files/s")
 		})
 
-		b.Run(fmt.Sprintf("LocalNeed100Largest@%d", size), func(b *testing.B) {
+		b.Run(fmt.Sprintf("n=LocalNeed100Largest/size=%d", size), func(b *testing.B) {
 			count := 0
 			for range b.N {
 				it, errFn := db.AllNeededGlobalFiles(folderID, protocol.LocalDeviceID, config.PullOrderLargestFirst, 100, 0)