fuzz_lex_test.nx

fuzz_lex_test.nx -- mutational fuzz against lex_source. Mutates valid NishiLang sources and feeds them to the lexer. The lexer should ALWAYS return without crashing -- malformed input may produce unexpected tokens, but it must never dereference NULL or run off the end of the buffer. This turns the theoretical "lex_source is robust" claim into an empirical "we've tried 10,000 mutated inputs and none crashed" result. If any run DOES crash, nx_assert fires and we get a clean post-mortem + the exact seed for replay. Seed corpus: "func f() { return 42 }" -- minimal function "let x: i64 = 100" -- let statement "0xdeadbeef + 0b1010" -- mixed-base integers "\"hello world\"" -- string literal "if a < b { x = 1 }" -- control flow "x[0].field" -- postfix chain "// comment\nfunc g() {}" -- comment handling "0.5 + 3.14f" -- floating-point literals