docs: Reorganize documentation into three-tier structure

**Three Manuals for Three Audiences:**

1. **User Manual** (docs/user/) - "Just Get It Done"
   - For end users, engineers, students
   - Simple, practical, step-by-step
   - Quick start, tutorials, examples, troubleshooting
   - Philosophy: Show me how to solve my problem

2. **Contributor Manual** (docs/contributor/) - "Show Me the Code"
   - For developers, contributors, advanced users
   - Technical, detailed, design rationale
   - Testing, architecture, performance, CI/CD
   - Philosophy: Explain HOW and WHY

3. **The JuliaFEM Book** (docs/book/) - "Let Me Show You How I Think"
   - For researchers, theory nerds, and Jukka
   - Comprehensive, educational, opinionated, personal
   - Math foundations, design philosophy, history, research
   - Philosophy: Mix theory, code, and personal experience

**Reorganization:**
- Moved: TESTING_PHILOSOPHY.md → contributor/testing_philosophy.md
- Moved: STATUS.md → contributor/status.md
- Moved: TEST_FIXES_NEEDED.md → contributor/test_fixes_needed.md
- Moved: lagrange_basis_functions.md → book/lagrange_basis_functions.md
- Moved: benchmarks/ → book/benchmarks/
- Created: docs/README.md (main index explaining structure)
- Created: README.md in each section explaining audience and contents
- Updated: All references in scripts and source files

**Naming:** All docs now lowercase (testing_philosophy not TESTING_PHILOSOPHY)

**Benefits:**
- Clear separation of concerns
- Users don't get overwhelmed with implementation details
- Contributors get technical depth
- Book preserves deep theory and personal insights
- Each manual optimized for its audience

**Next:** Populate each section with appropriate content

docs/contributor/status.md

@@ -0,0 +1,95 @@
+# JuliaFEM Revival Status - November 8, 2025
+
+## ✅ SUCCESS: Package Loads!
+
+JuliaFEM now loads successfully on Julia 1.12.1:
+```bash
+julia> using JuliaFEM
+✓ JuliaFEM loads successfully
+Exported names: 171
+```
+
+## Fixed Issues
+
+### 1. Element Type Signature Errors (CRITICAL)
+**Problem:** Element type changed from `Element{Basis}` to `Element{M, Basis} where M`  
+**Fixed in:**
+- `vendor/FEMBase.jl/src/FEMBase.jl` - Added AbstractBasis import
+- `vendor/FEMBase.jl/src/elements_lagrange.jl` - Fixed Poi1 subtyping
+- `vendor/FEMBeam.jl/src/beam3d.jl` - 3 function signatures
+- `vendor/MortarContact2D.jl/src/mortar2d.jl` - 2 functions
+- `vendor/MortarContact2D.jl/src/contact2d.jl` - 3 functions
+- `vendor/MortarContact2DAD.jl/src/mortar2dad.jl` - 1 function
+- `vendor/MortarContact2DAD.jl/src/contact2dad.jl` - 1 function
+- `src/problems_mortar_3d.jl` - 2 functions (M renamed to FS to avoid conflict)
+- `src/problems_contact_3d.jl` - 3 functions (M renamed to FS)
+- `src/io.jl` - 15 dispatch functions
+
+### 2. Merge Conflicts (Issue #250 from 2019)
+**Fixed in:**
+- `test/runtests.jl` - Removed conflict markers
+- `src/problems_elasticity.jl` - Resolved and simplified
+
+### 3. Missing Package Dependencies
+**Fixed:**
+- Created `vendor/MortarContact2DAD.jl/Project.toml`
+- Updated `Manifest.toml` to use local vendor packages
+
+### 4. Parallel Assembly Code
+**Fixed:**
+- Removed references to non-existent `problem.assemble_parallel` field
+- Simplified to use non-threaded assembly (threading can be added back later)
+
+## Test Status
+
+**Test Suite:** 5 passed, 51 errored (but package loads!)
+
+The errors are due to deeper API incompatibilities with Julia 1.12:
+- Method signature mismatches (e.g., `jacobian` function)
+- Some tests expect features from incomplete multithreading branch
+- API evolution over 6+ years (Julia 0.6 → 1.12)
+
+## What Works
+
+✅ Package installation and loading  
+✅ All vendor packages compile  
+✅ No type signature errors  
+✅ Core data structures intact  
+✅ 171 symbols exported  
+✅ Basic FEM infrastructure present
+
+## Next Steps for Full Revival
+
+1. **Fix jacobian/geometry method mismatches** - Update vendor/FEMBasis for Julia 1.12
+2. **Fix remaining test errors** - Systematic fixes for API changes
+3. **Add threading infrastructure** - Properly implement parallel assembly
+4. **Update documentation** - Reflect Julia 1.12 compatibility
+5. **Benchmark performance** - Establish baseline vs old version
+
+## Key Learnings
+
+- Multi-package ecosystems are maintenance nightmares (see llm/TECHNICAL_VISION.md)
+- Type stability critical: Dict-based fields caused 100× slowdown
+- Git history cleanup successful: 99MB → 9.8MB (90% reduction)
+- Vendor packages approach works for development
+
+## Files Modified
+
+**Critical fixes (this session):**
+- 10 source files with Element type fixes
+- 2 merge conflict resolutions  
+- 2 dependency files (Project.toml, Manifest.toml)
+- 1 assembly simplification
+
+**Scripts created:**
+- `test.sh` - Test runner
+- `fix_src_element_types.py` - Automated type fixing
+
+## Conclusion
+
+**Mission accomplished:** JuliaFEM loads on modern Julia! 
+
+While tests have errors, the **fundamental blocker (type signatures) is resolved**. 
+The package is now in a state where systematic fixing of remaining issues can proceed.
+
+The 51 test errors are fixable - they're API evolution issues, not architectural problems.