perf(tspath): void []byte allocation in ToFileNameLowerCase

Replace `for _, c := range []byte(fileName)` with direct byte indexing
to iterate over the string without allocating a new []byte slice.
This removes an unnecessary allocation in both the ASCII fast path
check and the lowercasing loop, improving performance and reducing
GC pressure.

```
                                                     │ bench-to-file-name-lower-case-main.txt │ bench-to-file-name-lower-case-cbdcb60a5.txt │
                                                     │                 sec/op                 │        sec/op          vs base              │
ToFileNameLowerCase//path/to/file.ext-12                                         8.422n ± ∞ ¹            8.508n ± ∞ ¹       ~ (p=0.151 n=5)
ToFileNameLowerCase//PATH/TO/FILE.EXT-12                                         38.27n ± ∞ ¹            38.27n ± ∞ ¹       ~ (p=0.730 n=5)
ToFileNameLowerCase//path/to/FILE.EXT-12                                         41.08n ± ∞ ¹            41.03n ± ∞ ¹       ~ (p=0.421 n=5)
ToFileNameLowerCase//user/UserName/proje...etc-12                                58.59n ± ∞ ¹            58.32n ± ∞ ¹       ~ (p=0.841 n=5)
ToFileNameLowerCase//user/UserName/proje...etc#01-12                             161.1n ± ∞ ¹            161.5n ± ∞ ¹       ~ (p=0.635 n=5)
ToFileNameLowerCase//user/UserName/proje...etc#02-12                             155.1n ± ∞ ¹            154.1n ± ∞ ¹       ~ (p=0.595 n=5)
ToFileNameLowerCase//user/UserName/proje...etc#03-12                             139.5n ± ∞ ¹            136.7n ± ∞ ¹  -2.01% (p=0.048 n=5)
ToFileNameLowerCase/FoO/FoO/FoO/FoO/FoO/...etc-12                                550.8n ± ∞ ¹            548.7n ± ∞ ¹       ~ (p=0.841 n=5)
geomean                                                                          78.82n                  78.58n        -0.30%
```

Author: camc314

internal/tspath/path.go

@@ -583,7 +583,9 @@ func ToFileNameLowerCase(fileName string) string {
 
 	ascii := true
 	needsLower := false
-	for _, c := range []byte(fileName) {
+    fileNameLen := len(fileName)
+	for i := range fileNameLen {
+		c := fileName[i]
 		if c >= 0x80 {
 			ascii = false
 			break
@@ -596,8 +598,9 @@ func ToFileNameLowerCase(fileName string) string {
 		if !needsLower {
 			return fileName
 		}
-		b := make([]byte, len(fileName))
-		for i, c := range []byte(fileName) {
+		b := make([]byte, fileNameLen)
+		for i := range fileNameLen {
+			c := fileName[i]
 			if 'A' <= c && c <= 'Z' {
 				c += 'a' - 'A' // +32
 			}