Fix unit mismatch in Dijkstra calc_index

[remete618]

Summary

• Fixes the implement Dijkstra maybe have issue #1318
• Removes the subtraction of meter-unit self.min_x/self.min_y from grid-index values node.x/node.y in calc_index

What changed

PathPlanning/Dijkstra/dijkstra.py, calc_index method:

# Before (mixes grid indices with meter-unit min values)
return (node.y - self.min_y) * self.x_width + (node.x - self.min_x)

# After (uses grid indices only)
return node.y * self.x_width + node.x

node.x and node.y are already grid indices (computed via calc_xy_index which divides by resolution). Subtracting self.min_x/self.min_y (in meters) is a unit mismatch. The algorithm produced correct results because the offset was constant across all nodes, preserving dictionary key uniqueness, but the formula was conceptually wrong.

Test plan

• Existing Dijkstra tests pass
• Path output is identical (the index offset was constant, so behavior is unchanged)