_displacement (mean per-keypoint delta between the last two track frames,
normalized by bbox height) was duplicated verbatim in
edge/detectors/fall.py and edge/detectors/immobility.py.
Move it to edge/geometry.py as track_displacement(track) and have both
detectors call the shared helper instead of their own private copy.
Pure refactor — behavior is identical (byte-identical formula), no
config or public API changes.
Tests: tests/test_geometry.py adds direct coverage for the new public
helper (fewer-than-two-frames default, and the normalized-by-height
calculation). Existing fall/immobility tests are unchanged and stay green,
confirming behavior parity.
