feat(vm): String Concatenation

[davydog187]

Wires up the .. operator end-to-end. The codegen was already emitting a :concatenate instruction but with the wrong shape (assumed consecutive registers) and the executor had no handler for it. This fixes the instruction to use two explicit source registers like every other binary op, and adds the executor handler with Lua's number-to-string coercion (1 .. 2 → "12").

Progress

• Phase 0: Arithmetic operations and single return
• Phase 1: Local variables with register assignment
• Phase 2: Global variables (get_global, set_global)
• Phase 3: Conditionals (if/elseif/else, and/or short-circuit)
• Phase 4: Loops (while, repeat, numeric for)
• Phase 5: Functions, closures, and upvalues
• Phase 6: Tables and method calls
• Phase 7: Native function calls (Lua→Elixir calling convention)
• Phase 8: Generic for, varargs, and multiple returns
• Phase 9: String concatenation and bitwise operations
• Phase 10: Value encoding/decoding (Lua.VM.Value)
• Phase 11: deflua integration (reimplement lib/lua.ex against VM.State)
• Phase 12: Peephole optimizer
• Phase 13: Remove Luerl dependency