- Pedagogical philosophy: theory before code, Lax equivalence foundation,
  error analysis throughout, progressive complexity, reproducibility
- Document content gaps from roadmap analysis: elliptic PDEs, high-order
  schemes, SBP/mimetic methods, curvilinear grids, adjoint methods
- Contribution guidance for each gap with SymPy/Devito examples
- Chapter template for new content structure
- Standards for SymPy math derivations, Devito solver patterns
- Code verification hierarchy (exact solutions, MMS, conservation)
- Plotting, image generation, and Quarto guidelines
- Testing, linting, and local development instructions
- Attribution and licensing requirements

The wave equation example in index.qmd used `Eq(u.dt2, c**2 * u.dx2)`
which generates invalid C code (assigns to an expression). Fixed to use
`solve()` to derive the explicit update stencil.

The diffusion example in what_is_devito.qmd used `alpha` and `dt` without
defining them. Fixed by adding parameter definitions and using the
`solve()` pattern for consistency.

Added test_book_code_verification.py to verify code examples are correct.

Tests verify Devito code examples from:
- devito_abstractions.qmd: Grid, Function, TimeFunction, 2D diffusion
- boundary_conditions.qmd: Dirichlet BCs, subdomain method, complete solver
- advec1D_devito.qmd: Upwind, Lax-Wendroff, Lax-Friedrichs schemes

All 14 new tests pass, ensuring code snippets in the book compile
and produce correct results.