v0.5.1

Fixes #9: round-trip mark drills ("set a mark and snap back") could be passed with an instant :q at 0 keystrokes, because the final cursor equals the start.

• round-trip motion challenges now carry a via waypoint, verified against vim's jumplist — you must actually make the far jump (G, `a, /search); a j/k shuffle or instant quit no longer counts
• quitting a motion drill with zero keystrokes is now an abstain (mastery untouched), not a graded attempt
• all 233 challenges re-verified in real vim under the new rule