Update DESIGN.md to reflect current migration status

[krystophny]

Objective

Update DESIGN.md to accurately reflect the current state of the text-to-AST migration and clarify the implementation status.

Required Updates

1. Update Implementation Status Section

• Mark Phase 1 as BLOCKED (waiting for fortfront)
• Mark Phase 2 as NOT STARTED (requires Phase 1)
• Add warnings about text-based code still present

2. Add Migration Status Table

## Current Migration Status

| Component | Current State | Target State | Status |
|-----------|--------------|--------------|--------|
| Rule Engine | Mixed text/AST | Pure AST | BLOCKED |
| F001-F005 | Text-based | AST-based | TODO |
| F006-F009 | Partially AST | Pure AST | TODO |
| Formatter | Not implemented | AST-based | WAITING |
| LSP Server | Skeleton only | Full implementation | FUTURE |
| Performance Rules | Not implemented | AST-based | TODO |

3. Add Critical Dependencies Section

## Critical Dependencies on fortfront

### Immediate Needs (Blocking)
- Static library (fortfront #416)
- Fortran modules (fortfront #417)
- Validation tests (fortfront #418)

### Future Needs (For formatter)
- CST implementation
- Trivia preservation
- Source reconstruction

4. Update Roadmap Section

• Reference new ROADMAP.md file
• Clarify phase dependencies
• Add timeline estimates

Definition of Done

• Current status accurately documented
• Migration progress clearly shown
• Dependencies explicitly listed
• Timeline updated
• References to ROADMAP.md added

Size: 1-2 hours

Priority: LOW (documentation update)

Notes

This helps developers understand the current state and what work remains. Should be updated periodically as migration progresses.