prop_lex_number_test.nx

prop_lex_number_test.nx -- property test for lex_source on numbers. Property: for every random decimal digit sequence D with value V, lex_source(D) produces exactly one TK_INT with int_val == V. Catches: * Overflow in the digit accumulator (V doesn't match hand math) * Off-by-one in digit iteration * Mis-tokenising legal digit chars as something else * Crash on edge cases (leading zeros, max i64 magnitude, etc.) Runs 2000 iterations. Each iteration picks a random number of digits (1..18 -- safely under i64 range for uniform digits).