Add Phase 3 content: ADER, Staggered Grids, and Attenuation Models

Chapter 8 additions:
- Section 8.4: ADER finite difference schemes (Taylor expansion in time)
- Section 8.5: Staggered grid formulations (velocity-pressure systems)

Chapter 7 additions:
- Section 7.4: Viscoacoustic waves with three rheological models
  (SLS, Kelvin-Voigt, Maxwell)
- Section 7.5: 3D Viscoelastic waves with Qp/Qs attenuation

New solvers:
- ader_devito.py: ADER acoustic solver with high-order time stepping
- staggered_devito.py: Staggered grid acoustic solver
- viscoacoustic_devito.py: Three viscoacoustic models
- viscoelastic_devito.py: Full 3D viscoelastic with TensorTimeFunction

91 new tests (411 total)

Author: ggorman

book-roadmap.md

@@ -1,7 +1,8 @@
 """High-Order Methods module for Finite Difference Computing with PDEs.
 
-This module provides dispersion analysis tools and Dispersion-Relation-Preserving
-(DRP) finite difference schemes for wave equation solvers.
+This module provides dispersion analysis tools, Dispersion-Relation-Preserving
+(DRP) finite difference schemes, ADER time integration, and staggered grid
+solvers for wave equations.
 
 Submodules
 ----------
@@ -13,6 +14,15 @@
     DRP wave equation solvers using Devito, with pre-computed and custom
     optimized coefficients.
 
+ader_devito
+    ADER (Arbitrary-order-accuracy via DERivatives) time integration for
+    the acoustic wave equation. Enables high-order temporal accuracy and
+    larger CFL numbers than standard leapfrog schemes.
+
+staggered_devito
+    Staggered grid acoustic wave solvers using the velocity-pressure
+    formulation. Supports 2nd and 4th order spatial discretization.
+
 Key Functions
 -------------
 fornberg_weights
@@ -23,6 +33,10 @@
     Compute custom DRP coefficients via optimization.
 solve_wave_drp
     Solve 2D wave equation with DRP scheme.
+solve_ader_2d
+    Solve 2D acoustic wave equation with ADER time integration.
+solve_staggered_acoustic_2d
+    Solve 2D acoustic wave equation with staggered grid scheme.
 dispersion_ratio
     Compute velocity error ratio for a FD scheme.
 
@@ -39,6 +53,26 @@
 ...     use_drp=True
 ... )
 
+ADER solver with high CFL number:
+
+>>> from src.highorder import solve_ader_2d
+>>> result = solve_ader_2d(
+...     extent=(1000., 1000.),
+...     shape=(101, 101),
+...     c_value=1.5,
+...     courant=0.85,  # Higher CFL than leapfrog
+... )
+
+Staggered grid solver:
+
+>>> from src.highorder import solve_staggered_acoustic_2d
+>>> result = solve_staggered_acoustic_2d(
+...     extent=(2000., 2000.),
+...     shape=(81, 81),
+...     velocity=4.0,
+...     space_order=4,
+... )
+
 Dispersion analysis:
 
 >>> from src.highorder import fornberg_weights, dispersion_ratio
@@ -47,6 +81,16 @@
 >>> print(f"Velocity ratio: {ratio:.4f}")
 """
 
+from src.highorder.ader_devito import (
+    ADERResult,
+    biharmonic,
+    compare_ader_vs_staggered,
+    graddiv,
+    gradlap,
+    gradlapdiv,
+    lapdiv,
+    solve_ader_2d,
+)
 from src.highorder.dispersion import (
     analytical_dispersion_relation,
     cfl_number,
@@ -72,26 +116,46 @@
     solve_wave_drp_1d,
     to_full_stencil,
 )
+from src.highorder.staggered_devito import (
+    StaggeredResult,
+    compare_space_orders,
+    convergence_test_staggered,
+    dgauss_wavelet,
+    solve_staggered_acoustic_2d,
+)
 
 __all__ = [
     "DRP_COEFFICIENTS",
     "FORNBERG_COEFFICIENTS",
+    "ADERResult",
+    "StaggeredResult",
     "WaveDRPResult",
     "analytical_dispersion_relation",
+    "biharmonic",
     "cfl_number",
+    "compare_ader_vs_staggered",
     "compare_dispersion_wavefields",
+    "compare_space_orders",
     "compute_drp_weights",
+    "convergence_test_staggered",
     "critical_dt",
+    "dgauss_wavelet",
     "dispersion_difference",
     "dispersion_error",
     "dispersion_ratio",
     "drp_coefficients",
     "drp_objective_tamwebb",
     "fornberg_weights",
+    "graddiv",
+    "gradlap",
+    "gradlapdiv",
+    "lapdiv",
     "max_frequency_ricker",
     "numerical_dispersion_relation",
     "nyquist_spacing",
     "ricker_wavelet",
+    "solve_ader_2d",
+    "solve_staggered_acoustic_2d",
     "solve_wave_drp",
     "solve_wave_drp_1d",
     "to_full_stencil",