Fix ModelingToolkit v10 build compatibility issues

Resolves compilation errors and uses proper MTK v10 syntax patterns:

**Key Fixes:**

1. **Removed problematic @mtkcompile macro usage with let blocks**
   - @mtkcompile doesn't work with let blocks in MTK v10
   - Replaced with @nAmed + structural_simplify pattern

2. **Fixed system completion requirements**
   - MTK v10 requires all systems to be completed before ODEProblem creation
   - Added structural_simplify() to all system definitions
   - Used @nAmed macro for systems that need names

3. **Updated syntax patterns:**
   - Simple systems: `@named sys_raw = ODESystem(eqs, t); sys = structural_simplify(sys_raw)`
   - Complex systems: `let ... structural_simplify(@nAmed tempSys = ODESystem(eqs, t)) end`
   - Equation definitions: separated from system compilation for clarity

4. **Working systems verified:**
   - SA systems (sa1sys-sa4sys): ✓ Compiled successfully
   - SB systems (sb1sys-sb2sys): ✓ Compiled successfully
   - SC systems (sc1sys-sc2sys): ✓ Compiled successfully
   - ND system (nd1sys): ✓ Compiled with stack overflow fix
   - NA systems (na1sys-na2sys): ✓ Simplified syntax working

**Project Dependencies:**
- ModelingToolkit requirement: v10 only (no backward compatibility)
- Removed IfElse.jl dependency (deprecated in MTK v10)

**Status:**
- Core benchmark systems now compile successfully with MTK v10
- Basic solve functionality verified
- Complex systems (nc5, etc.) may need additional work for edge cases

This resolves the main ODESystem constructor compatibility issues.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas