feat: add tree-sitter syntax highlighting module

- Cargo.toml: Add tree-sitter-highlight + grammar crates
  (rust, bash, json). TOML dropped due to API incompatibility.
- src/highlight.rs: Language enum, highlight_code() function,
  4 unit tests covering parsing and HTML generation.
- Uses static HTML_ATTRS array for zero-allocation rendering.

Cargo.toml

@@ -11,3 +11,9 @@ maud           = "0.26"
 pulldown-cmark = "0.12"
 thiserror      = "2"
 walkdir        = "2"
+
+# Syntax highlighting
+tree-sitter-bash      = "0.23"
+tree-sitter-highlight = "0.24"
+tree-sitter-json      = "0.24"
+tree-sitter-rust      = "0.23"