This PR modifies mesa_signals to move from a class-based descriptor pattern (Computable/Computed) to a standard Python property factory (@computed).

Currently, accessing a derived property triggers the Computable.__get__ descriptor, which then fetches a separate Computed object to execute the logic. This adds unnecessary function call depth to every attribute access.

I suggest two streamlined components:

1. @computed Decorator: A factory that returns a standard Python property. The "reactive" logic (dependency tracking, dirty checking) is captured in the property's closure, eliminating the descriptor lookup overhead.
2. ComputedState: A lightweight, internal-only class using __slots__ to hold the value, dirty flag, and dependencies.

• ComputedProperty: A custom property class to identify computed properties.

API Changes (Before vs. After)

Before (Current)

class MyAgent(HasObservables):
    wealth = Computable()  # Descriptor

    def __init__(self):
        super().__init__()
        # Manual binding required
        self.wealth = Computed(self._calculate_wealth)

    def _calculate_wealth(self):
        return self.cash + self.assets

After

class MyAgent(HasObservables):
    @computed
    def wealth(self):  # Clean, standard Python syntax
        return self.cash + self.assets

Addresses (2) from #3131