SteadyHands: track the fingertip directly, drop cursor offset

#15 in Riparion/riparion-retro — merged 2026-06-09

The on-target ring is large enough that the finger doesn't occlude it, so the sniper-scope offset (cursor floating above the fingertip) just made the control feel indirect. Remove it — the cursor now tracks the finger, and the goal is simply to keep your finger inside the ring.

Changes

• Remove the cursor_offset prop (no caller set it).
• target_position spans the full [MARGIN, 1 - MARGIN] band on both axes again — no need to reserve vertical room for the offset's reach. Dropped its now-moot "within reach" test.
• Cursor render and hit test use the finger position directly.
• Refresh on-screen hints and doc comments accordingly.

All minigames-kit SteadyHands tests pass.

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