perf(tspath): avoid string copy in ToFileNameLowerCase (#1575)

Author: camc314

internal/tspath/path.go

@@ -5,6 +5,7 @@ import (
 	"slices"
 	"strings"
 	"unicode"
+	"unsafe"
 
 	"github.com/microsoft/typescript-go/internal/stringutil"
 )
@@ -613,7 +614,17 @@ func ToFileNameLowerCase(fileName string) string {
 			}
 			b[i] = c
 		}
-		return string(b)
+		// SAFETY: We construct a string that aliases b’s backing array without copying.
+		// (1) Lifetime: The address of b’s elements escapes via the returned string,
+		//     so escape analysis allocates b’s backing array on the heap. The string
+		//     header points to that heap allocation, ensuring it remains live for the
+		//     string’s lifetime.
+		// (2) Initialization: We assign to every b[i] before creating the string.
+		//     (Note: Go zeroes all allocated memory, so “uninitialized” bytes cannot occur.)
+		// (3) Immutability: We do not modify b after this point, so the string view
+		//     observes immutable data.
+		// (4) Non-empty: On this path len(b) > 0, so &b[0] is a valid, non-nil pointer.
+		return unsafe.String(&b[0], len(b))
 	}
 
 	return strings.Map(func(r rune) rune {