Add automatic fine aiming and fix cue rail clearance

[darrinm]

Small pointer movements can swing soft shots too far, and the cue can intersect the table rail while pulled back. This adds automatic fine aiming on mouse and touch and keeps the cue above the cushions and rails.

• Keep direct dragging for rough aim and power. Holding within 6 pixels for 400 ms enters fine aim with power locked and a visible indicator and cue highlight.
• Move across the cue to rotate at 0.1° per pixel. Moving 20 pixels along the cue resumes direct aiming without a jump, including side-on and diagonal shots.
• Retract the cue along its elevated axis and account for cue thickness, spin, and rail bevels when calculating clearance.
• Keep the immediate first-impact guide horizontal, starting at the cue ball. Update the in-game help and README.

Validation: full unit suite, 26 physics validation checks, and production build. Browser checks cover automatic entry, cue-relative fine adjustment on mobile-size tables, smooth exit, indicator cleanup, cue placement on all four sides, and agreement between displayed aim and the released shot. Geometry tests check cue clearance across directions, pull distances, spin, and corner positions.