Replace flat 400 XP/level formula with the segmented curve from
LEVEL_RULES: Lv.1-5 steep ramp, Lv.6-10 moderate, Lv.11-20 linear +80,
Lv.21-35 +120, Lv.36-50 +180. Level 50 is hard-capped with xpToNextLevel=0.
Uses binary search over pre-computed cumulative thresholds for O(log n)
level lookup.
Add 11 new test cases covering challenge session creation, correct/wrong
answers, idempotent duplicate submission, completion settlement, resource
exhaustion, Plus user bypass, and invalid input validation.
Refactor test helpers to use queue-based mockImplementation pattern for
more reliable db.select mocking across complex async flows.
