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.

doc/site/modules/core/string.markdown

@@ -120,6 +120,15 @@ Returns the index of the first byte matching `search` in the string or `-1` if
 
 It is a runtime error if `search` is not a string.
 
+### **indexOf**(search, start)
+
+Returns the index of the first byte matching `search` in the string or `-1` if
+`search` was not found, starting a byte offset `start`. The start can be
+negative to count backwards from the end of the string.
+
+It is a runtime error if `search` is not a string or `start` is not an integer
+index within the string's byte length.
+
 ### **iterate**(iterator), **iteratorValue**(iterator)
 
 Implements the [iterator protocol](../../control-flow.html#the-iterator-protocol)