Parses a tag string to extract the numeric size in billions. Returns -1 if the tag cannot be parsed.
Returns double to handle decimal values like "0.6b" and "1.7b". If all tags are integers, this could be simplified to return int.
Examples: - "1b" -> 1.0 - "4b" -> 4.0 - "12b" -> 12.0 - "27b" -> 27.0 - "235b" -> 235.0 - "0.6b" -> 0.6 - "1.7b" -> 1.7 - "1b-it-qat" -> 1.0 (extracts number before 'b') - "1b-it-q4_K_M" -> 1.0 (extracts number before 'b') - "12b-it-qat" -> 12.0 (extracts number before 'b')