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/src/lib.rs

@@ -1,5 +1,7 @@
 // JSON tool call filtering for display (moved from g3-core)
 pub mod filter_json;
+pub mod syntax_highlight;
+pub mod streaming_markdown;
 
 use anyhow::Result;
 use crossterm::style::{Color, ResetColor, SetForegroundColor};