Fix mixed coordinate units in Dijkstra grid indexing

[huxiaoyi-ovo]

Reference issue

Fixes #1318.

What does this implement/fix?

DijkstraPlanner.calc_index() previously subtracted the map's world-coordinate origin (min_x, min_y, in metres) from Node.x and Node.y, even though these node coordinates are already zero-based grid indices (in cells). This mixes units and produces an incorrect nonzero index for the grid-origin cell whenever the world origin is offset.

Use the direct row-major index node.y * self.x_width + node.x. The previous offset was constant for a map, so the fix corrects the index definition without changing Dijkstra's search ordering or planned paths.

A focused regression test checks two maps with different nonzero world origins and non-unit resolutions (0.5 and 2.0 m/cell), including:

• mapping the world origin to grid cell (0, 0) and index 0;
• mapping an interior cell to y * width + x;
• mapping the final grid cell to width * height - 1.

This also addresses the maintainer's earlier request for an offset-origin, non-unit-resolution regression test on #1360 (closed without merge).

Additional information

Validated against current upstream master (cdd0cc888802b584c2d654ca85e3c5460973487d) using fork-only draft PR #2:

• Linux Python 3.14 full CI — passed.
• Windows Python 3.14 full CI — passed.
• macOS Python 3.14 full CI — passed.
• CodeQL code scanning — passed.
• Diff: only PathPlanning/Dijkstra/dijkstra.py and tests/test_dijkstra.py, with no new dependencies.

CheckList

• Did you add an unittest for your new example or defect fix?
• Did you add documents for your new example? (Not applicable: corrects an internal grid-index formula; no public API or algorithm documentation changes.)
• All CIs are green? (Fork CI: Linux, Windows, macOS, CodeQL; upstream checks will run separately.)

AI assistance was used to review the issue history and prepare the minimal fix and regression test; the final branch was checked against upstream, and the full fork CI passed.