Add D-infinity flow length computation (#1012)

New flow_length_dinf() computes flow path lengths from D-inf angle
grids using Tarboton's two-neighbor angle decomposition. Downstream
mode gives the proportion-weighted distance to outlet; upstream mode
gives the longest path from any divide.

All four backends: NumPy, CuPy, Dask+NumPy, Dask+CuPy.
25 tests covering correctness, edge cases, and cross-backend consistency.

Also adds the missing flow_accumulation_dinf entry to the README
feature matrix.

Author: brendancol

README.md

@@ -293,7 +293,10 @@ In the GIS world, rasters are used for representing continuous phenomena (e.g. e
 | [Flow Direction (Dinf)](xrspatial/flow_direction_dinf.py) | Computes D-infinity flow direction as a continuous angle toward the steepest downslope facet | Tarboton 1997 | ✅️ | ✅️ | ✅️ | ✅️ |
 | [Flow Direction (MFD)](xrspatial/flow_direction_mfd.py) | Partitions flow to all downslope neighbors with an adaptive exponent (Qin et al. 2007) | Qin et al. 2007 | ✅️ | ✅️ | ✅️ | ✅️ |
 | [Flow Accumulation (D8)](xrspatial/flow_accumulation.py) | Counts upstream cells draining through each cell in a D8 flow direction grid | Jenson & Domingue 1988 | ✅️ | ✅️ | ✅️ | ✅️ |
+| [Flow Accumulation (Dinf)](xrspatial/flow_accumulation_dinf.py) | Accumulates upstream area by splitting flow proportionally between two neighbors (Tarboton 1997) | Tarboton 1997 | ✅️ | ✅️ | ✅️ | 🔄 |
 | [Flow Accumulation (MFD)](xrspatial/flow_accumulation_mfd.py) | Accumulates upstream area through all MFD flow paths weighted by directional fractions | Qin et al. 2007 | ✅️ | ✅️ | ✅️ | 🔄 |
+| [Flow Length (D8)](xrspatial/flow_length.py) | Computes D8 flow path length from each cell to outlet (downstream) or from divide (upstream) | Standard (D8 tracing) | ✅️ | ✅️ | ✅️ | 🔄 |
+| [Flow Length (Dinf)](xrspatial/flow_length_dinf.py) | Proportion-weighted flow path length using D-inf angle decomposition (downstream or upstream) | Tarboton 1997 | ✅️ | ✅️ | ✅️ | 🔄 |
 | [Watershed](xrspatial/watershed.py) | Labels each cell with the pour point it drains to via D8 flow direction | Standard (D8 tracing) | ✅️ | ✅️ | ✅️ | ✅️ |
 | [Basins](xrspatial/watershed.py) | Delineates drainage basins by labeling each cell with its outlet ID | Standard (D8 tracing) | ✅️ | ✅️ | ✅️ | ✅️ |
 | [Stream Order](xrspatial/stream_order.py) | Assigns Strahler or Shreve stream order to cells in a drainage network | Strahler 1957, Shreve 1966 | ✅️ | ✅️ | ✅️ | ✅️ |

docs/source/reference/hydrology.rst

@@ -53,6 +53,13 @@ Flow Length
 
     xrspatial.flow_length.flow_length
 
+Flow Length (D-infinity)
+========================
+.. autosummary::
+    :toctree: _autosummary
+
+    xrspatial.flow_length_dinf.flow_length_dinf
+
 Flow Path
 =========
 .. autosummary::

xrspatial/__init__.py

@@ -45,6 +45,7 @@
 from xrspatial.flow_direction_dinf import flow_direction_dinf  # noqa
 from xrspatial.flow_direction_mfd import flow_direction_mfd  # noqa
 from xrspatial.flow_length import flow_length  # noqa
+from xrspatial.flow_length_dinf import flow_length_dinf  # noqa
 from xrspatial.flow_path import flow_path  # noqa
 from xrspatial.focal import mean  # noqa
 from xrspatial.glcm import glcm_texture  # noqa