The previous implementation of List.+ used conditional checks (`this.count > 0`, `that.count > 0`) before iterating, which caused the loop body to be skipped entirely for empty lists. This commit removes those guards, allowing the for-in loops to run unconditionally — the Wren VM's iterate/iterateValue protocol already handles the empty case correctly at the native level. Additionally, the `list_iterate` native in `wren_core.c` now returns `false` immediately when the list count is zero and the iterator argument is null, preventing an out-of-bounds index of 0 from being returned for empty lists. The test file `test/list/concat.wren` is renamed to `test/list/plus.wren` and extended with a check that the original list `a` remains unmodified after concatenation. A new test in `test/list/iterate.wren` verifies that iterating over an empty list returns `false`.
Rename the IO.write method to IO.print, which now prints without a trailing newline, and add a new IO.print method that appends a newline after output. Update all benchmark, example, and test files to use IO.print instead of IO.write. Additionally, implement Unicode escape sequence parsing in the compiler's string tokenizer, supporting \uXXXX and \u{...} syntax with proper UTF-8 encoding for code points up to U+10FFFF.
Replace the old validateIndex function with validateNum and validateInt helpers that produce descriptive runtime error messages. Add new test files covering invalid index types, out-of-bounds indices, and iterator validation for list insert, iterate, and iteratorValue methods. Rename existing number operator error tests to use consistent "must be a number" phrasing.
