Use FutureWarning for legacy API deprecations

FutureWarning is shown by default; DeprecationWarning is filtered.
Users need to see these warnings to migrate before 1.0.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: waltsims

docs/get_started/first_simulation.rst

@@ -158,7 +158,7 @@ Now we combine all four components and run:
    ``kspaceFirstOrder2D`` / ``3D`` functions. See :doc:`/get_started/new_api`
    for details.
 
-   The legacy functions still work but emit deprecation warnings.
+   The legacy functions still work but emit warnings.
 
 Step 6: Visualize Results
 -------------------------

docs/get_started/new_api.rst

@@ -134,4 +134,4 @@ arguments:
    result = kspaceFirstOrder(kgrid, medium, source, sensor, **kwargs)
 
 The legacy ``kspaceFirstOrder2D`` and ``kspaceFirstOrder3D`` functions
-continue to work but emit ``DeprecationWarning``.
+continue to work but emit ``FutureWarning``.

kwave/kspaceFirstOrder2D.py

@@ -205,7 +205,7 @@ def kspaceFirstOrder2D(
         "kspaceFirstOrder2D is deprecated. Use kspaceFirstOrder() from "
         "kwave.kspaceFirstOrder instead. See kwave.compat.options_to_kwargs() "
         "for migrating options.",
-        DeprecationWarning,
+        FutureWarning,
         stacklevel=2,
     )
 

kwave/kspaceFirstOrder3D.py

@@ -198,7 +198,7 @@ def kspaceFirstOrder3D(
         "kspaceFirstOrder3D is deprecated. Use kspaceFirstOrder() from "
         "kwave.kspaceFirstOrder instead. See kwave.compat.options_to_kwargs() "
         "for migrating options.",
-        DeprecationWarning,
+        FutureWarning,
         stacklevel=2,
     )