Add comprehensive stress tests for streaming markdown formatter

Add 10 stress tests covering:
- Nested formatting (bold in italic, italic in bold)
- Empty/minimal content edge cases
- Escape sequences and special characters
- Lists with complex inline formatting
- Links with various content types
- Tables with formatting in cells
- Code blocks (should not format contents)
- Mixed block elements (headers, quotes, rules)
- Nested lists (3+ levels, mixed types)
- Pathological/adversarial inputs (unbalanced delimiters, unicode, long lines)

All 45 tests pass.

crates/g3-cli/Cargo.toml

@@ -28,6 +28,8 @@ crossterm = "0.29.0"
 ratatui = "0.29"
 termimad = "0.34.0"
 regex = "1.10"
+syntect = "5.3"
+once_cell = "1.19"
 
 [dev-dependencies]
 tempfile = "3.8"