fix(math): geomLineIntersection returns null when segments share a start point

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Problem

geomLineIntersection returned null when a0 and b0 are the same point, even though the intersection is well-defined (it's that shared point).

Root cause: when a0 === b0, both qpx and qpy are 0, so uNumerator = 0. The condition:

if (uNumerator && denominator) {

treated a falsy uNumerator as a reason to skip the check entirely — but u = 0 is a perfectly valid intersection parameter (the intersection is at t = 0, u = 0, i.e. the shared start point).

Fix

Only gate on denominator (which being zero means the segments are parallel). A zero uNumerator simply means u = 0, which is within the valid [0, 1] range.

// before
if (uNumerator && denominator) {

// after
if (denominator) {

Test

Added a regression test using real-world geographic coordinates where a0 and b0 are coincident.