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/.
