TrackTools: document TrackGeometry physics motivation and C++ usage

Document TrackGeometry as the first component of the shared
TrackTools package.

Explain the impact of geometrically shortened TPC tracks on
tracking efficiency, fake associations, dE/dx PID resolution,
biases and comparisons between data, MC and reconstruction passes.

Document the purpose of the three geometry methods:
- L: candidate fast per-stack linear approximation.
- H: analytic helix reference for precision studies.
- K: O2 propagation reference.

Provide practical user examples:
- C++ analysis with two configurable sector-edge guard widths.
- Verified interactive ROOT session using a complete 5D track.
- Independent available-row selections and H/L comparisons.
- Python/AliasDataFrame visualization with synthetic primary
  and secondary trajectories.

Document the committed portable tests, execution instructions,
geometry conventions and the distinction between predicted
available rows and measured clusters.

Include the synthetic geometry figure as supplementary QA.
Detailed physics-validation findings remain in the project CRR.

Author: miranov25