feat: implement documentation comments

Add support for doc comments (/// syntax) that can be attached to
declarations for documentation purposes. The implementation:

- Adds DocComment token kind to lexer
- Recognizes /// as doc comment syntax (distinct from // regular comments)
- Parses consecutive doc comments and combines them into a single string
- Adds doc field to FunctionDecl, TypeDecl, LetDecl, EffectDecl, TraitDecl
- Passes doc comments through parser to declarations
- Multiple consecutive doc comment lines are joined with newlines

This enables documentation extraction and could be used for generating
API docs, IDE hover information, and REPL help.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/ast.rs

@@ -227,6 +227,8 @@ pub enum Declaration {
 #[derive(Debug, Clone)]
 pub struct FunctionDecl {
     pub visibility: Visibility,
+    /// Documentation comment (from /// doc comments)
+    pub doc: Option<String>,
     pub name: Ident,
     pub type_params: Vec<Ident>,
     pub params: Vec<Parameter>,
@@ -251,6 +253,8 @@ pub struct Parameter {
 /// Effect declaration
 #[derive(Debug, Clone)]
 pub struct EffectDecl {
+    /// Documentation comment
+    pub doc: Option<String>,
     pub name: Ident,
     pub type_params: Vec<Ident>,
     pub operations: Vec<EffectOp>,
@@ -270,6 +274,8 @@ pub struct EffectOp {
 #[derive(Debug, Clone)]
 pub struct TypeDecl {
     pub visibility: Visibility,
+    /// Documentation comment
+    pub doc: Option<String>,
     pub name: Ident,
     pub type_params: Vec<Ident>,
     /// Optional version annotation: type User @v2 { ... }
@@ -342,6 +348,8 @@ pub struct HandlerImpl {
 #[derive(Debug, Clone)]
 pub struct LetDecl {
     pub visibility: Visibility,
+    /// Documentation comment
+    pub doc: Option<String>,
     pub name: Ident,
     pub typ: Option<TypeExpr>,
     pub value: Expr,
@@ -352,6 +360,8 @@ pub struct LetDecl {
 #[derive(Debug, Clone)]
 pub struct TraitDecl {
     pub visibility: Visibility,
+    /// Documentation comment
+    pub doc: Option<String>,
     pub name: Ident,
     /// Type parameters: trait Functor<F> { ... }
     pub type_params: Vec<Ident>,