2 Hours: Hand-Crafting a Structural Mechanics Solver with AI

Please create a project-level specification file CLAUDE.md for our 2D Frame Structural Mechanics Solver (NSStructure 2D):

1. Civil Engineering Standards:
   - Bending Moment Diagram (BMD) MUST be strictly drawn on the TENSION side;
   - Global coordinate conventions: +X to the right, +Y upward, counterclockwise rotation is positive;
   - Axial Force Diagram (AFD): Tension and compression in distinct blue/red colors;
   - Shear Force Diagram (SFD): Plotted perpendicular to element axis.
2. Architecture Separation:
   - Data Layer (/src/types/ & /src/store/): Pure reactive state for nodes, elements, loads;
   - Solver Layer (/src/core/): Pure TypeScript Direct Stiffness Method with NO DOM/UI imports;
   - View Layer (/src/components/): Canvas/SVG coordinate transforms and interactive UI.
3. Unit Test Validation:
   - All solver edits must pass simply supported beam analytical benchmark cases.

Implement the 2D Direct Stiffness Method FEM solver in /src/core/solver.ts:
1. 6x6 Local Element Stiffness Matrix [k_local] with axial (EA/L) and bending stiffness (12EI/L^3, 6EI/L^2, 4EI/L, 2EI/L);
2. Transformation Matrix [T] using element inclination angle θ;
3. Global Stiffness Matrix [K_global] assembly by degree of freedom indices;
4. Boundary condition enforcement (elimination or penalty method);
5. Solve [K]{Δ} = {P} for node displacements, element internal forces (N, V, M), and reactions.

Render structural analysis results in-place on Canvas:
1. BMD: Strictly rendered on the TENSION side of members. Under UDL, sample 20 points along member length for quadratic parabola with translucent blue fill;
2. SFD: Render rectangular/trapezoidal hatching perpendicular to member;
3. AFD: Tension in dark blue, compression in orange-red;
4. Deformed Shape: Hermite cubic interpolation with a user-adjustable magnification factor slider.