feat: add ast and parser implementation

PLAN.md

@@ -15,15 +15,13 @@ A Rust-flavored, C-targeting language - built pipeline-first.
 
 ## Phase 2 - Parser
 
-- [ ] Write grammar for the base subset
-  - `fn` declarations, `return`, `let`, int/bool literals
-  - Arithmetic (`+`, `-`, `*`, `/`), comparison (`==`, `!=`, `<`, `>`)
-  - Function call expressions
-- [ ] Implement recursive-descent parser
-- [ ] Build typed AST: `FnDecl`, `Block`, `ReturnStmt`, `LetStmt`, `BinExpr`, `CallExpr`, `Literal`, `Ident`
-- [ ] Attach source spans to every AST node
-- [ ] Emit structured parse errors with span info
-- [ ] Unit-test: parse valid snippets, expect correct AST shapes
+- [x] Write grammar for the base subset
+  - `fn` declarations, `return`, int/bool literals
+  - Arithmetic (`+`, `-`, `*`, `/`)
+- [x] Implement recursive-descent parser
+- [x] Attach source spans to every AST node
+- [x] Emit structured parse errors with span info
+- [x] Unit-test: parse valid snippets, expect correct AST shapes
 
 ## Phase 3 - Semantic Analysis