Add explicit length parameters to wrenStringConcat to support binary-safe string concatenation, with -1 sentinel for automatic strlen calculation. Implement wrenStringFind using Boyer-Moore-Horspool algorithm for efficient substring search, returning UINT32_MAX when needle not found. Update all callers in wren_core.c validation functions and wren_value.c metaclass creation to pass length arguments. Extend string/index_of.wren tests with edge cases including empty needle, full match, and complex overlapping patterns.