fix: harden analysis geometry diagnostics

[Gilbert243]

Summary

Hardens the numerical analysis geometry layer after the AgencityLab 1.1.1 validation campaign.

What changes

• makes curvature diagnostics robust to numerically unresolved velocity near slow/turning points;
• preserves exact zero curvature for sampled trajectories that are collinear to machine resolution;
• keeps curvature undefined (NaN) when the discrete velocity is not numerically resolved;
• makes leading/trailing invalid valid_mask samples equivalent to slicing the contiguous valid interval;
• preserves NaN for discontinuous masks where phase unwrapping across omitted samples is ambiguous;
• adds regression tests for scale/rotation robustness, turning points, constant trajectories, circle curvature, sinusoidal motion on a complex line, explicit speed tolerance, and valid-mask semantics.

Scientific scope

This change is diagnostic/numerical only. It does not modify the canonical Agencity equations, CRM, M, O, D, S, J, Theta, beta, P_c, or b.

Local validation

• targeted tests: 27 passed
• full suite: 635 passed
• Ruff: passed
• mypy: passed (227 source files)
• package build: succeeded