Add Phase 2 numerical verification foundation

[Kandil2001]

What changed

• adds a CMake/CTest verification build without replacing the existing solver workflow
• introduces reusable 2D field, compatible discrete-operator, Poisson, and convergence-contract components
• verifies D(G(phi)) = L(phi) in the interior for the selected operator pair
• adds a manufactured Poisson problem on 17×17, 33×33, and 65×65 grids
• checks approximately second-order spatial convergence
• checks that RBGS and RBSOR reach the same discrete solution
• adds explicit solver statuses and consecutive convergence checks
• adds GCC/Clang, Debug/Release, sanitizer-enabled GitHub Actions coverage
• documents the Phase 2 convergence contract and the next solver-integration step

Why

The current 36-case study records completed executions, but those cases reached their configured maximum outer-iteration limits. This PR creates the verification and convergence foundation needed before changing the production pressure-correction solver or regenerating results.

Impact

The existing src/lid_cavity.cpp, current CSV results, figures, scripts, and main branch behavior are unchanged. This is an additive foundation intended for review before numerical integration.

Validation performed

• Release CMake build: passed
• CTest: 3/3 passed
• AddressSanitizer + UndefinedBehaviorSanitizer build: passed
• Sanitized CTest: 3/3 passed

Next step after review

Integrate the convergence tracker and dimensionless residual definitions into the existing C++ cavity solver, then tune and converge only the canonical N=32, Re=100, upwind, RBSOR case before changing the wider study.