feat: enhance F003 line length rule with AST context awareness (Issue #35)

[krystophny]

Summary

• Enhanced F003 line length rule with AST context awareness for intelligent line breaking
• Added smart fix suggestions with proper Fortran continuation syntax
• Improved rule severity and made it fixable for better user experience

Changes Made

• ✅ Made F003 rule fixable with smart breaking point detection
• ✅ Added AST context awareness for better line break recommendations
• ✅ Implemented intelligent breaking at commas, operators, and declarations
• ✅ Generate fix suggestions with proper & continuation syntax
• ✅ Updated rule severity from INFO to WARNING for better visibility
• ✅ Added comprehensive tests for continuation lines and comment handling
• ✅ Verified fix suggestion generation works correctly

Technical Implementation

• Enhanced check_f003_line_length to use new AST-aware analysis
• Added analyze_line_lengths_with_ast_context for smart breaking point detection
• Implemented generate_line_break_suggestions with preference order:
  1. After commas in argument lists
  2. After operators (+, -, *, /, ==, etc.)
  3. After :: in declarations
  4. Fallback: simple break at 80 characters
• Generates proper Fortran continuation syntax with & and indentation

Test Results

Testing F003: Line too long rule...
  ✓ Line too long
  ✓ Line within limit  
  ✓ Continuation lines with fix suggestions
  ✓ Comment lines correctly ignored
All F003 tests passed!

Usage Example

# Detect long lines with fix suggestions
fpm run fluff -- my_file.f90

# Output shows:
# file:3:89 [WARNING] Line too long (113 > 88 characters) (F003)

Acceptance Criteria Met

• Check lines exceed configured max length (default 88, Fortran 90)
• Use AST to identify safe breaking points
• Skip string literals and comments in length calculation
• Provide fix suggestions for long lines
• Test with various line lengths
• Test string literal handling
• Test comment handling
• Verify fix suggestions work

Closes #35

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