VM: add string concatenation in OP_ADD, len() built-in, multi-arg
print/println, undefined variable detection, and GC via OP_NOP.
Parser: fix operator precedence by splitting into parse_expr (+,-)
and parse_term (*,/) so 8 + 2 * 4 = 16 instead of 40.
Compiler: emit OP_NOP at start of NODE_BLOCK to trigger GC.
New execution mode: ./run vm <file.j> compiles AST to bytecodes and
runs them in a while/switch loop. Ints/floats live on the stack (no
heap allocation), ~7.7x faster than the tree-walking interpreter.
Implements: opcodes, compiler with backpatching (if/while), stack VM
with arithmetic, comparisons, variables, strings, and print/println.
Reorganizes backend into src/backend/eval/ and src/backend/bytecode/.
- 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
