Function sanitize 

pub fn sanitize(branch: &str) -> String

Sanitize a branch name so it can be used as a KuzuDB table name prefix.

Rules applied (in order):

• / → __ (preserves branch hierarchy visibility)
• any remaining non-alphanumeric char → _
• leading digit → prefix with b_ (table names can’t start with a digit)

Examples:

• main → main
• feat/auth → feat__auth
• feat/auth-v2 → feat__auth_v2
• release/v1.0 → release__v1_0