```python
"""A word-to-line index for a plain-text note."""

import re

WORD_RUN = re.compile(r"[a-z0-9]+")


def words_of_line(line: str) -> list:
    """Return the maximal runs of ASCII letters and digits in the lowercased line."""
    return WORD_RUN.findall(line.lower())


def build_word_index(text: str) -> dict:
    """Build a mapping from each word to the strictly increasing list of line numbers."""
    if not isinstance(text, str):
        raise ValueError("text must be a string")
    
    index = {}
    rows = text.split("\n")
    for line_num, line in enumerate(rows, start=1):
        words = words_of_line(line)
        if words:
            unique_words = set(words)
            for word in unique_words:
                if word not in index:
                    index[word] = []
                index[word].append(line_num)
    return index
```