Fix kde dask backends dropping points on descending-coordinate templates (#3627)

[brendancol]

Closes #3627

• _filter_points_to_tile computed the tile extent as tile_x0 .. tile_x0 + cols*dx, which inverts when dx/dy are negative. Descending-coordinate templates (the normal north-up raster layout) lost most or all of their points before the kernel ran: compact kernels returned all zeros, gaussian returned partially wrong values. The fix orders the tile edges with min/max before widening by the cutoff.
• Affected dask+numpy and dask+cupy only. The eager paths were already fixed for this in Fix KDE all-zero output with descending-coordinate templates #1199; the tile filter sits above them and was missed.
• The sweep state CSV update for the kde accuracy audit rides along in this commit.

Backends: numpy and cupy unchanged (already correct); dask+numpy and dask+cupy fixed and now match numpy exactly on descending-y, descending-x, and both-descending templates.

Test plan:

• New test_dask_matches_numpy_descending_coords (3 orientations x 3 kernels) fails on main, passes with the fix
• New TestDaskCupyDescending runs the dask+cupy path on a GPU host (CUDA verified locally)
• Full test_kde.py suite: 65 passed
• Repro script from kde returns all-zero or wrong output on dask backends with descending-coordinate templates #3627 shows max|diff|=0 across all orientations after the fix

[brendancol]

PR Review: Fix kde dask backends dropping points on descending-coordinate templates (#3627)

Blockers (must fix before merge)

• none

Suggestions (should fix, not blocking)

• xrspatial/tests/test_kde.py (TestDaskCupyDescending): the dask+cupy regression test covers only the descending-y quartic case. The filter is shared code, so this is enough to pin the bug, but a desc-x case would be cheap and would catch any future divergence between the two dask wrappers (they duplicate the tile loop).

Nits (optional improvements)

• xrspatial/kde.py:391-401: tile_x1 = tile_x0 + tile_cols * dx overshoots the last pixel centre by one spacing, so the filter keeps a slightly wider band than strictly needed. That is the safe direction (never drops a contributing point) and matches the pre-existing behavior on ascending grids; worth a half-sentence in the comment if anyone touches this again.

What looks good

• The fix is minimal: order the tile edges with min/max before widening by the cutoff, exactly parallel to what #1199 did inside the kernels.
• Verified locally on a CUDA host: dask+numpy and dask+cupy now match numpy exactly (max abs diff 0 for quartic, fringe-only for gaussian) on desc-y, desc-x, and desc-both templates. Before the fix, compact kernels returned all zeros and gaussian returned partially wrong values.
• The new parametrized test (3 orientations x 3 kernels) fails on main and passes here; tolerance choices mirror the existing #1199 regression tests (exact for compact kernels, atol=1e-5 for the gaussian 4*bw box-cutoff fringe).
• The sweep-accuracy state CSV update rides along per the sweep contract; it does not touch code.

Checklist

• Algorithm matches reference/paper (filter now consistent with the kernels' own cutoff logic)
• All implemented backends produce consistent results (verified numpy / cupy / dask+numpy / dask+cupy on this host)
• NaN handling is correct (unchanged by this PR; tracked separately in #3628)
• Edge cases are covered by tests (3 orientations x 3 kernels + dask+cupy)
• Dask chunk boundaries handled correctly (per-tile origins already sign-correct; only the filter was wrong)
• No premature materialization or unnecessary copies
• Benchmark exists or is not needed (bug fix, no perf-relevant change: the filter does the same amount of work)
• README feature matrix updated (n/a, no API change)
• Docstrings present and accurate (helper docstring unchanged, still correct)

[brendancol]

PR Review follow-up (#3627)

Disposition of the earlier findings, after ae16e5e:

• Suggestion (dask+cupy desc-x coverage): fixed. TestDaskCupyDescending is now parametrized over desc-y and desc-x; 66 tests pass locally, both GPU variants executed on this host.
• Nit (filter overshoot comment): fixed. The comment now says the tile edge overshoots the last pixel centre by one spacing and why that is the safe direction.

No new findings. No blockers.