feat: add DISTANCE mode with traces and leaderboard (PR 4)

[AlexBodner]

Summary

• Adds DISTANCE analytics mode: direction dots (same as DIRECTION) plus cumulative distance chips (m), per-player trace lines on the radar minimap, and a 3-second distance leaderboard end-card. No m/s badges.
• Distance integrates via gated pitch homography on VideoTrackingSession.tracks.
• Extracts shared _render_speed_distance_traces(..., show_speed=) in distance.py (spotlight + speed overlay hooks for PR5).

Stack: PR 1 → PR 2 → PR 3 (base) → this PR

Demo

DISTANCE mode — speed + distance chips, trace minimap, end-card (08fd33_0.mp4, truncated preview).

Download full demo clip

Homography / kinematics design

• VideoTrackingSession.tracks — lazy collect_tracks + compute_kinematics using _gated_speed_transforms() (not gap-filled).
• session.gap_filled_transforms_by_frame — instant speed badges (same as SPEED).
• session.minimap_transforms_by_frame — warp feet for trace buffer on radar.
• valid_pitch_cm() — filters trace outliers before drawing polylines.

Scope

• examples/soccer/distance.py — shared trace renderer + run_distance
• kinematics.py — PlayerTrack, compute_kinematics, cumulative_distance_at_frame
• motion.py — draw_distance_labels, build_trace_minimap, draw_distance_end_card, dim_frame, spotlight
• speed.py — only_tid filter on _speed_by_tid
• Mode.DISTANCE wired in main.py + README section

Not included: run_all, SPEED_AND_DISTANCE wiring (PR5), GK goal-distance locks, goal shading (PR6).

Test plan

• Merge PR 3 first (rebase if PR 3 changes during review)
• python main.py --source_video_path data/2e57b9_0.mp4 --target_video_path data/renders/2e57b9_0-distance.mp4 --mode DISTANCE --max-frames 90 --tracker bytetrack
• Verify distance chips, trace minimap, and end-card leaderboard
• Confirm SPEED and DIRECTION modes unchanged