Add while loops, GC mark-and-sweep, and malloc block reuse

- Lexer: add INDENT/DEDENT tokens, <, >, : operators, while keyword,
  closing DEDENT emission, include guards
- Parser: add NODE_WHILE with while_loop union, parse while/cond/body
  blocks, include guards
- Eval: add while loop evaluation, GC integration with roots from env,
  debug and gc flags, <, > comparison operators
- GC: implement mark-and-sweep collector with 3 stages (mark roots,
  sweep unmarked, join free blocks)
- Allocator: block reuse via first-fit search with splitting, exponential
  heap growth, NULL check on malloc, include guards, marked field in metadata
- Object: add include guards, fix include to use allocator.h

src/main.c

@@ -34,7 +34,10 @@ int main(int argc, char **argv) {
   ast_debug(block);
   
   Environment env = {0};
-  eval(block, &env, allocPtr);
+  eval(block, &env, allocPtr, 0, 1);
+
+  printf("heapSize=%zu\n", allocPtr->size);
+  // JLANG_visualize(allocPtr);
 
   return 0;
 }