feat: add stat instance method to File class returning Stat object

Implement `File.stat` method that performs an asynchronous `fstat` call on the open file descriptor, returning a `Stat` object populated with file metadata (device, inode, mode, link count, user, group, size, block size, block count). Refactor the existing `statPathCallback` into a generic `statCallback` shared by both `statPath` and the new `fileStat` function. Bump `MAX_METHODS_PER_CLASS` from 11 to 12 to accommodate the new foreign method slot. Add comprehensive test coverage including successful stat retrieval and error handling when called on a closed file.

test/io/file/size.wren

@@ -1,5 +1,4 @@
 import "io" for File
-import "scheduler" for Scheduler
 
 var file = File.open("test/io/file/file.txt")
 System.print(file.size) // expect: 19

test/io/file/stat.wren

@@ -0,0 +1,18 @@
+import "io" for File, Stat
+
+var file = File.open("test/io/file/file.txt")
+var stat = file.stat
+
+System.print(stat is Stat)            // expect: true
+System.print(stat.device is Num)      // expect: true
+System.print(stat.inode is Num)       // expect: true
+System.print(stat.mode is Num)        // expect: true
+System.print(stat.linkCount)          // expect: 1
+System.print(stat.user is Num)        // expect: true
+System.print(stat.group is Num)       // expect: true
+System.print(stat.specialDevice)      // expect: 0
+System.print(stat.size)               // expect: 19
+System.print(stat.blockSize is Num)   // expect: true
+System.print(stat.blockCount is Num)  // expect: true
+
+file.close()

test/io/file/stat_after_close.wren

@@ -0,0 +1,6 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.close()
+
+file.stat // expect runtime error: File is not open.