feat(math): add katex-rs math rendering module

Add src/math.rs with render_math() wrapper around katex-rs for
build-time LaTeX to HTML conversion. Wire module into main.rs.

- Use KatexContext::default() for standard LaTeX function registry
- Settings: display_mode toggle, throw_on_error=false for graceful
  degradation
- Include unit tests for inline/display math rendering

Cargo.toml

@@ -32,5 +32,6 @@ tree-sitter-yaml       = "0.7"
 lightningcss = "1.0.0-alpha.70"
 
 # Config parsing
-serde = { version = "1", features = ["derive"] }
-toml  = "0.8"
+katex-rs = "0.2.3"
+serde    = { version = "1", features = ["derive"] }
+toml     = "0.8"