feat: add negative start support and validate index in String.indexOf(_,_)

Simplify arithmetic in wrenStringFind by removing startIndex offset from loop range and using start directly as the initial index. Rename internal primitives to string_indexOf1 and string_indexOf2 for clarity. Add validateIndex helper to clamp negative start values and reject out-of-bounds indices. Update documentation to describe the new start parameter behavior. Add comprehensive test suite for indexOf with start, including negative offsets, edge cases, and error conditions.

test/core/string/index_of.wren

@@ -5,12 +5,6 @@ System.print("abcd".indexOf("abcd")) // expect: 0
 System.print("abcd".indexOf("abcde")) // expect: -1
 System.print("abab".indexOf("ab")) // expect: 0
 
-System.print("abcd".indexOf("cd", 0)) // expect: 2
-System.print("abcd".indexOf("cd", 1)) // expect: 2
-System.print("abcd".indexOf("cd", 2)) // expect: 2
-System.print("abcd".indexOf("cd", 3)) // expect: -1
-System.print("abcd".indexOf("cd", 10)) // expect: -1
-
 // More complex cases.
 System.print("abcdefabcdefg".indexOf("defg")) // expect: 9
 System.print("abcdabcdabcd".indexOf("dab")) // expect: 3

test/core/string/index_of_start.wren

@@ -0,0 +1,39 @@
+// An empty string is anywhere you look for it.
+System.print("abcd".indexOf("", 0)) // expect: 0
+System.print("abcd".indexOf("", 1)) // expect: 1
+System.print("abcd".indexOf("", 2)) // expect: 2
+
+// Overlapping results.
+System.print("aaaaa".indexOf("aaaa", 0)) // expect: 0
+System.print("aaaaa".indexOf("aaaa", 1)) // expect: 1
+System.print("aaaaa".indexOf("aaaa", 2)) // expect: -1
+
+// It's OK if the needle extends past the end.
+System.print("abcd".indexOf("abcde", 0)) // expect: -1
+System.print("abcd".indexOf("cde", 3)) // expect: -1
+
+System.print("abcd".indexOf("cd", 0)) // expect: 2
+System.print("abcd".indexOf("cd", 1)) // expect: 2
+System.print("abcd".indexOf("cd", 2)) // expect: 2
+System.print("abcd".indexOf("cd", 3)) // expect: -1
+
+// Negative start.
+System.print("abcd".indexOf("cd", -4)) // expect: 2
+System.print("abcd".indexOf("cd", -3)) // expect: 2
+System.print("abcd".indexOf("cd", -2)) // expect: 2
+System.print("abcd".indexOf("cd", -1)) // expect: -1
+
+// Skips past earlier results.
+System.print("here as well as here".indexOf("here", 1)) // expect: 16
+
+// Non-ASCII. Note that it returns byte indices, not code points.
+System.print("søméஃthîng".indexOf("e", 2)) // expect: -1
+System.print("søméஃthîng".indexOf("m", 2)) // expect: 3
+System.print("søméஃthîng".indexOf("thî", 8)) // expect: 9
+
+// 8-bit clean.
+System.print("a\0b\0c".indexOf("\0", 0)) // expect: 1
+System.print("a\0b\0c".indexOf("a", 0)) // expect: 0
+System.print("a\0b\0c".indexOf("b\0c", 1)) // expect: 2
+System.print("a\0b\0c".indexOf("a\0b\0c\0d", 0)) // expect: -1
+System.print("a\0b\0a\0b".indexOf("a\0b", 0)) // expect: 0

test/core/string/index_of_start_not_int.wren

@@ -0,0 +1 @@
+"abcd".indexOf("bc", 12.34) // expect runtime error: Start must be an integer.

test/core/string/index_of_start_not_num.wren

@@ -0,0 +1,2 @@
+"abcd".indexOf("bc", "not num") // expect runtime error: Start must be a number.
+System.print("after")

test/core/string/index_of_start_too_large.wren

@@ -0,0 +1 @@
+"abcd".indexOf("bc", 4) // expect runtime error: Start out of bounds.

test/core/string/index_of_start_too_small.wren

@@ -0,0 +1 @@
+"abcd".indexOf("bc", -5) // expect runtime error: Start out of bounds.