fix(rhumb-bearing): return NaN for coincident points

[JSap0914]

Bug

When start and end points are identical (coincident), rhumbBearing() returns 0 (north). This is incorrect — a bearing is geometrically undefined for a zero-length segment. Returning 0 silently implies a northward direction where there is none.

Closes #2478

Root Cause

calculateRhumbBearing computes Math.atan2(deltaLambda, deltaPsi). For coincident points both values are 0, and Math.atan2(0, 0) === 0 in JavaScript — producing a spurious bearing of 0°.

The Geodesy library that this implementation is adapted from (latlon-spherical.js) explicitly checks for this case and returns NaN.

Fix

Added an early-exit guard in calculateRhumbBearing:

if (from[0] === to[0] && from[1] === to[1]) {
  return NaN;
}

This matches the Geodesy reference behaviour and is consistent with what the regular bearing function also produces for coincident points (since Math.atan2(0,0) behaviour is identical there).

Verification

cd packages/turf-rhumb-bearing
pnpm run test:tape

Output:

TAP version 13
# bearing
ok 1 pair1
ok 2 coincident points return NaN
ok 3 coincident points return NaN (final bearing)
ok 4 invalid point

1..4
# tests 4
# pass  4

# ok

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