🧩 Propositional Logic Builder: Truth Tables, Parse Trees & Natural Deduction

Interactive propositional-logic constructor: click variable (A/B/C) and operator buttons (AND/OR/NOT/XOR/IMPLIES/IFF) to build a formula, or load a preset (De Morgan, contraposition, excluded middle, majority). A recursive-descent parser builds the AST, which renders live as a canvas parse tree, a full truth table with tautology/contradiction verdict, and a step-revealing natural-deduction proof (Modus Ponens, Modus Tollens, Hypothetical/Disjunctive Syllogism) with rule labels.