- 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
