Restore snapshot iteration to avoid iterator invalidation in type propagation

[trufae]

Motivation

• Fix a high-severity iterator invalidation in register-argument type propagation where iterating directly over &fcn->vars could be mutated by var_retype() -> r_anal_var_set_type() -> shadow_var_struct_members(), causing out-of-bounds access or crashes.

Description

• Replace the unsafe R_VEC_FOREACH(&fcn->vars, ...) with a stable snapshot obtained via r_anal_var_vec(anal, fcn, R_ANAL_VAR_KIND_REG), iterate over that snapshot, and free it after use, removing the now-unnecessary in-loop kind check.

Testing

• Attempted a local build with make -j but it failed in this environment due to a missing libr/config.mk, so a full build/test run could not be performed here.

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Codex Task