Fixes chain mastery prompt level logic for probe sessions

[ajlouie]

Initial probes (i.e., before any training sessions):

• 3 consecutive probe successes on a step should set that step to mastered.
• Fewer than 3 probe successes on a step should set that step to not_yet_started (or focus for the first unmastered step).

After initial probes:

• 3 consecutive probe failures on a not_yet_started or focus step should have no effect on target prompt level.

After mastery:

• 3 consecutive probe failures on a mastered or booster_mastered step should set the step status to booster_needed with shadow target prompt level.
• 3 consecutive probe failures on a booster_needed step should have no effect on target prompt level.