level_for_xp implements the two-segment level formula from
ARCHITECTURE.md §13. xp_for_win = base 50 + linearly-scaled speed bonus
(10..=50 for sub-2-minute wins) + 25 if no undo was used. PlayerProgress
exposes add_xp returning the previous level so callers can detect
level-up events.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
funman300