feat: Add AST emitter and comment support

Introduces a new AST emitter module responsible for serializing AST
nodes back into Myc source code. This emitter preserves leading comments
and whitespace, enabling a round-trip guarantee for the parser and
emitter.

The documentation in `docs/BNF.md` has been updated to reflect the new
syntax rules for comments, including single-line comments (`;`) and
documentation comments (`;;`). The rules clarify that comments must be
whole lines and that inline comments are disallowed.

The lexer and AST node structures have been updated to accommodate
`CommentLine` variants, allowing for different types of comments
(regular, documentation, and blank lines) to be stored and processed.

A new test suite `tests/comment_roundtrip.rs` has been added to verify
the round-trip property of the parser and emitter across all example
`.myc` files. This test ensures that parsing source code, emitting it,
and then parsing the emitted code again results in the same output
string.

The `ast.rs` binary now includes a `--emit` flag that uses the new
emitter to output the parsed source code, facilitating debugging and
testing of the emitter itself.

src/bin/ast.rs

@@ -1,5 +1,7 @@
 use clap::Parser;
+use myc::ast::compiler::emitter;
 use myc::ast::environment::Environment;
+use myc::ast::parser::Parser as MycParser;
 use myc::utils::tester;
 use std::fs;
 use std::io::{self, IsTerminal, Read};
@@ -43,6 +45,10 @@ struct Cli {
     /// Start MCP server (stdio transport, for LLM integration)
     #[arg(long)]
     mcp: bool,
+
+    /// Emit the parsed source back as Myc code (round-trip test)
+    #[arg(long)]
+    emit: bool,
 }
 
 fn main() {
@@ -108,7 +114,17 @@ fn main() {
     };
 
     if let Some(content) = source {
-        if cli.dump {
+        if cli.emit {
+            let mut parser = MycParser::new(&content);
+            let syntax_ast = parser.parse_expression();
+            if parser.diagnostics.has_errors() {
+                for diag in &parser.diagnostics.items {
+                    eprintln!("ERROR: {}", diag.message);
+                }
+                std::process::exit(1);
+            }
+            print!("{}", emitter::emit(&syntax_ast));
+        } else if cli.dump {
             match env.dump_ast(&content) {
                 Ok(dump) => println!("{}", dump),
                 Err(e) => eprintln!("Error dumping AST: {}", e),