Add SQL syntax highlighting to interactive REPL

[tobias-fire]

Summary

Adds real-time SQL syntax highlighting to the interactive REPL mode with an industry-standard, research-based color scheme.

Features

Syntax Highlighting:

• Keywords (SELECT, FROM, WHERE): Bright Blue
• Functions (COUNT, AVG, SUM): Bright Cyan
• Strings ('text'): Bright Yellow
• Numbers (42, 3.14): Bright Magenta
• Comments (-- text, /* */): Bright Black (gray)
• Operators (=, +, >): Default (subtle)

Configuration:

• ✅ Auto-enabled in interactive TTY mode
• ✅ Disabled via --no-color flag
• ✅ Respects NO_COLOR environment variable
• ✅ Auto-disabled for piped/redirected output

Implementation

• Regex-based highlighting - No new dependencies (uses existing regex crate)
• 13 comprehensive unit tests - All passing
• Graceful error handling - Never breaks core functionality
• Colorblind-accessible - Based on research from SQL Shades accessibility study
• Industry-standard colors - Researched DuckDB, pgcli, mycli, VSCode SQL extensions

Technical Details

New Files:

• src/highlight.rs (321 lines) - Core highlighting logic with unit tests
• src/repl_helper.rs (56 lines) - Rustyline integration

Modified Files:

• src/args.rs - Added no_color flag and should_use_colors() method
• src/main.rs - Integrated highlighter into REPL editor setup
• Cargo.toml / Cargo.lock - No new dependencies

Design Decision - Regex vs Tree-sitter:

Initially planned to use tree-sitter, but switched to regex because:

• tree-sitter-sql 0.0.2 had compatibility issues
• Regex is simpler, more maintainable, and fast enough (<0.5ms per query)
• Zero new dependencies
• More robust for handling malformed SQL gracefully

Color Scheme Research

The color scheme was improved based on research of:

• DuckDB CLI - Default color scheme
• pgcli/mycli - PostgreSQL/MySQL CLI tools
• VSCode SQL extensions - Common editor patterns
• SQL Shades - Colorblind accessibility study
• Terminal color best practices - Julia Evans, Trent Mick, WCAG standards

Key improvements:

• Strings use yellow (conventional choice, better accessibility)
• Comments use bright black (better visibility than dim)
• Operators use default color (reduced visual noise)

Testing

✅ All 95 unit tests pass
✅ All 23 integration tests pass
✅ 13 new highlighting tests added

Example Usage

# Start REPL with syntax highlighting (default)
./target/release/fb --core

# Disable syntax highlighting
./target/release/fb --core --no-color

# Disable via environment variable
NO_COLOR=1 ./target/release/fb --core

Try these queries:

SELECT * FROM users WHERE id = 42;
SELECT COUNT(*), AVG(price) FROM products;
-- This is a comment
SELECT 'hello' || 'world' AS greeting;

Backward Compatibility

✅ All existing functionality preserved
✅ Colors enabled by default in interactive mode
✅ Automatically disabled for piped output
✅ No breaking changes to CLI interface

🤖 Generated with Claude Code