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.

docs/BNF.md

@@ -66,6 +66,19 @@ This document defines the syntax (BNF) and semantics of the Myc language, a Lisp
 ```
 *(Lexical Note: `<ident-start>` consists of alphabetic characters or `+-*/<=>!$%&_?.`. `<ident-char>` includes digits as well. Identifiers evaluating to a field accessor begin with a `.` followed by one or more valid identifier characters.)*
 
+### Comments
+
+```ebnf
+<comment>     ::= ";"  " "? <line-text> <newline>
+<doc-comment> ::= ";;" " "? <line-text> <newline>
+```
+
+**Rules:**
+- Comments must be **whole lines** — they start at the beginning of a line. Inline comments (after code on the same line) are a compile error.
+- Comment lines directly preceding an expression form a **comment block** attached to that expression.
+- Blank lines between comment lines are preserved as separators within the block. The first and last line of a block must not be blank.
+- `;;` (doc comment) before a `def` or `macro` registers the text in the symbol documentation registry (available via `--dump-docs`). Before other expressions it is allowed but has no registry effect.
+
 ## 2. Core Semantics & Data Types
 
 - **Integers and Floats:** Standard 64-bit numeric types.
@@ -138,7 +151,8 @@ The Myc runtime environment provides a collection of built-in functions and macr
 ```clojure
 (do
   (def user {:id 101 :name "Alice" :role :admin})
-  (def role (.role user)) ; Evaluates to :admin
+  ; Evaluates to :admin
+  (def role (.role user))
 )
 ```
 
@@ -156,7 +170,8 @@ The Myc runtime environment provides a collection of built-in functions and macr
   (def prices (.price my_ticks))
 
   ;; Series indexing: 0 is the newest (15.5), 1 is the previous (11.2)
-  (+ (prices 0) (prices 1)) ;; Output: 26.7
+  ;; Output: 26.7
+  (+ (prices 0) (prices 1))
 )
 ```
 
@@ -223,6 +238,19 @@ The `check_syntax` tool uses the single-expression compiler pass. Wrap multiple
 (do (def x 1) (+ x 2))
 ```
 
+### Inline comments are not allowed
+
+Comments must be **whole lines** starting with `;` or `;;`. Placing a comment after code on the same line is a compile error.
+
+```clojure
+;; WRONG — inline comment causes a compile error:
+(def x 42) ; this is x
+
+;; CORRECT — comment on its own line before the expression:
+; this is x
+(def x 42)
+```
+
 ### Field accessors are functions, not property syntax
 
 ```clojure
@@ -247,7 +275,8 @@ user.name
         acc
         (again (- n 1) (* acc n)))))
 
-  (factorial 10 1) ;; Output: 3628800
+  ;; Output: 3628800
+  (factorial 10 1)
 )
 ```
 
@@ -258,7 +287,8 @@ user.name
   (macro unless [c body]
     `(if ~c ... ~body))
 
-  (unless false "executed") ;; Output: "executed"
+  ;; Output: "executed"
+  (unless false "executed")
 )
 ```
 
@@ -276,6 +306,7 @@ user.name
 
   ;; Read back the two most recent prices
   (def p (.price ticks))
-  (+ (p 0) (p 1)) ;; newest + second-newest
+  ;; newest + second-newest
+  (+ (p 0) (p 1))
 )
 ```