Add two binary strings

You are given two binary strings `a` and `b` (each consisting only of characters `'0'` and `'1'`). Return their sum as a binary string. ## Input - `a`: string, length `1..10^5` - `b`: string, length `1..10^5` ## Output - A string representing `a + b` in base-2, with no leading zeros unless the result is exactly `"0"`. ## Constraints / Notes - You may not convert the entire strings to built-in integers (they may exceed 64-bit limits). - Time: `O(|a| + |b|)`; extra space should be `O(|a| + |b|)` (or better). ## Example - `a = "1011"`, `b = "110"` → output: `"10001"`