fuzz: add printf_fuzz target

[chaliy]

Summary

Add a fuzz target for the printf builtin (builtins/printf.rs). It parses format strings with conversion specifiers and performs numeric conversions.

Scope

• Fuzz arbitrary format strings with random arguments
• Test edge cases in width/precision specifiers, numeric conversion, escape sequences

Priority

Low — simpler parser, but format string handling has known edge case history.