Full implementation of core "why not promoted" algorithm

[stereotype441]

(Parent issue #44897)

a42244f added basic functionality to the CFE and analyzer to show reasons for a promotion failure, but it only addressed two narrow corner cases:

• Erroneous access to a property due to the target being nullable, where the target is a local variable that was promoted but then later demoted due to a write.
• Erroneous access to a property due to the target being nullable, where the target is itself a property or field (and hence not promotable).

This logic should be expanded to address all scenarios where promotion might fail.