feat: add Stdin.readByte() method for reading single byte from stdin

Implement a new `readByte()` static method on the `Stdin` class that reads one byte from standard input, blocking the current fiber until a byte is received. The method returns the byte value as a number or `null` if stdin is closed.

Refactor the internal buffering mechanism by introducing a shared `read_()` helper method that both `readByte()` and `readLine()` use, replacing the previous per-method buffering logic. The `readByte()` implementation peels off the first byte from the internal `__buffered` string using `bytes[0]` and slicing.

Add two test files: `read_byte.wren` verifying byte-by-byte reading of "first" and "second" input lines, and `read_byte_eof.wren` confirming that reading after stdin is closed returns the expected error message.

test/io/stdin/read_byte.wren

@@ -0,0 +1,21 @@
+import "io" for Stdin
+
+for (i in 1...13) {
+  System.print(Stdin.readByte())
+}
+
+// stdin: first
+// expect: 102
+// expect: 105
+// expect: 114
+// expect: 115
+// expect: 116
+// expect: 10
+
+// stdin: second
+// expect: 115
+// expect: 101
+// expect: 99
+// expect: 111
+// expect: 110
+// expect: 100