Locate First Singular Character

You are given a string s consisting of lowercase English letters. Your task is to identify the first character in the string that appears exactly once (i.e., has no duplicates anywhere else in the string) and return its 0-based index position.

If every character in the string repeats at least once, meaning no singular (non-repeating) character exists, return -1.

A character is considered singular if and only if its frequency throughout the entire string is exactly one. The goal is to find the earliest occurring such character based on its position in the string.

Key Observations:

• The string contains only lowercase English letters ('a' to 'z'), providing a fixed alphabet size of 26.
• You need to return the index of the first singular character, not the character itself.
• The answer must reflect the leftmost position among all singular characters.
• If multiple characters appear exactly once, return the index of the one that appears first in the string.