Challenge 3: Method vs. Associated Function, Explained — Possible Solution
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The defining difference is whether the function takes self as its
first parameter. A METHOD (like area(&self)) takes self, which means
it needs an EXISTING INSTANCE to operate on — self IS that instance,
borrowed or owned depending on which form (&self, &mut self, or self)
is used. An ASSOCIATED FUNCTION (like new(radius: f64) -> Circle) has
no self parameter at all — it isn't tied to any particular instance,
it's just a function that happens to live inside the type's own impl
block, namespaced under that type.

WHY Struct::new(...) CAN'T BE CALLED AS instance.new(...): dot syntax
(instance.method()) only works for functions that actually accept an
instance as their first argument — under the hood, rect.area() is
essentially sugar for Rectangle::area(&rect), automatically passing
rect as the self argument. new() has no self parameter to fill that
role at all — there's no instance to automatically pass in, because
call to new() is what PRODUCES the instance in the first place. Trying
instance.new(...) would be asking Rust to pass an already-existing
instance into a function whose signature doesn't have any parameter
slot for one — a fundamental syntactic mismatch, not just a style
restriction, which is exactly why associated functions are always
called via Type::function_name(...) instead.