fix(oop): assign the return value to the method name in Capacity

[Poseidonas]

Fixes #38.

Capacity computes the percentage into the protected _capacity member and stops there. In ST a method returns by assigning to its own name, so it hands back 0.

That return value is read:

src/program.st:41   capacity_percentage := tank.Capacity(currentVolume := current_volume);
src/program.st:85   capacity_percentage := tank.Capacity(currentVolume := current_volume);
src/program.st:109  capacity_percentage := tank.Capacity(currentVolume := current_volume);

and the exercise text asks for:

Capacity : REAL that returns a real with the percentage of capacity of the volume of the tank.

So capacity_percentage stays at zero for anyone following the module, which is a confusing thing to meet while learning the language.

The member assignment is kept and the return added on top, which is the shape GetState already uses in ValveBase.st:

GetState := ValveState#Open;
...
_state := GetState;

Applied in both copies — exercises/solution/ and exercises/4_inheritance_complex_valve/.

Two things I looked at and left alone

0_basic_valve_class/src/ValveBase.st also has a GetState that does not assign to its name, but every method there is ; — it is the skeleton the exercise asks the reader to fill in, so it is correct as it stands.

TankWithShape.st has two VolumeCalculator : REAL methods which assign to volume rather than to the method name. Here I was not sure what you intend: the slides say the volume is "calculated with a method called VolumeCalculator and the result stored in the property volume", and the only caller ignores the return value:

CalculatorWithRegulation.st:42   tankCube.VolumeCalculator();

So the body matches the brief and nothing is broken — but then the : REAL return type is never used, which is its own small trap for a reader learning how ST methods return. Either adding VolumeCalculator := volume; or dropping the return type would settle it. Happy to do whichever you prefer, in this PR or a separate one.

On verification

I have no AX toolchain here, so I did not compile this. What I checked is that the change follows the assignment form already used elsewhere in the module, that the callers listed above do read the value, and that nothing else in module 6 has the same shape — I walked every METHOD ... : <type> in the module and the only remaining ones are the two described above.

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