refactor: Move theory docs to src/ with lowercase naming

- Moved docs/theory/lagrange_basis_functions.md → src/lagrange_basis_functions.md
- Updated all references in scripts and source files
- Using lowercase for consistency (no uppercase in filenames)
- Documentation now under src/ for automated doc generation

Rationale: Documentation should be close to implementation and follow
consistent naming conventions (lowercase).

scripts/README.md

@@ -35,7 +35,7 @@ julia --project=. scripts/generate_lagrange_basis.jl
 - `src/basis/lagrange_generated.jl` (commit this file!)
 
 **Theory:**
-See `docs/theory/lagrange_basis_functions.md` for mathematical foundation.
+See `src/lagrange_basis_functions.md` for mathematical foundation.
 
 **Architecture:**
 
@@ -130,6 +130,6 @@ Pre-compute stiffness matrices for simple elements.
 
 **See also:**
 
-- `docs/theory/lagrange_basis_functions.md` - Mathematical theory
+- `src/lagrange_basis_functions.md` - Mathematical theory
 - `src/basis/lagrange_generator.jl` - Symbolic generation engine
 - `llm/VISION_2.0.md` - Overall project architecture

scripts/generate_lagrange_basis.jl

@@ -21,7 +21,7 @@
 #   - After modifying lagrange_generator.jl
 #
 # THEORY:
-#   See docs/theory/lagrange_basis_functions.md for full mathematical details
+#   See src/lagrange_basis_functions.md for full mathematical details
 #
 # ==============================================================================
 
@@ -617,7 +617,7 @@ println(output, "#   cd /path/to/JuliaFEM.jl")
 println(output, "#   julia --project=. scripts/generate_lagrange_basis.jl")
 println(output, "#")
 println(output, "# Theory:")
-println(output, "#   See docs/theory/lagrange_basis_functions.md")
+println(output, "#   See src/lagrange_basis_functions.md")
 println(output, "#")
 println(output, "# Generator:")
 println(output, "#   src/basis/lagrange_generator.jl (symbolic engine)")

src/basis/lagrange_generator.jl

@@ -45,7 +45,7 @@
 #   - Version control shows what changed
 #
 # SEE:
-#   - docs/theory/lagrange_basis_functions.md (mathematical explanation)
+#   - src/lagrange_basis_functions.md (mathematical explanation)
 #   - scripts/generate_lagrange_basis.jl (generation script)
 #   - src/basis/lagrange_generated.jl (output - do not edit manually!)
 #