perf: right-size register tuple allocations

Replace the hard-coded 256-register top-level allocation and the
+64 over-allocation on every callee call with values derived from
the prototype's max_registers field.

- vm.ex: proto.max_registers + 16 (was 256)
- executor.ex: max(max_registers, param_count) + 16 (was + 64)

The +16 buffer accounts for multi-return expansion slots that the
codegen doesn't always track in max_registers. This cuts register
tuple waste significantly for recursive workloads like fib(N).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: davydog187

lib/lua/vm.ex

@@ -16,9 +16,10 @@ defmodule Lua.VM do
   """
   @spec execute(Prototype.t(), State.t()) :: {:ok, list(), State.t()}
   def execute(%Prototype{} = proto, state \\ State.new()) do
-    # Create register file - tuple of nils
-    # For now, just allocate 256 registers (we'll optimize this later)
-    registers = Tuple.duplicate(nil, 256)
+    # Create register file sized to the prototype's needs.
+    # The +16 buffer covers multi-return expansion slots that the codegen doesn't
+    # always track in max_registers (call results can land beyond the stated max).
+    registers = Tuple.duplicate(nil, proto.max_registers + 16)
 
     # Execute the prototype instructions
     {results, _final_regs, final_state} =

lib/lua/vm/executor.ex

@@ -32,7 +32,7 @@ defmodule Lua.VM.Executor do
   @spec call_function(term(), list(), State.t()) :: {list(), State.t()}
   def call_function({:lua_closure, callee_proto, callee_upvalues}, args, state) do
     callee_regs =
-      Tuple.duplicate(nil, max(callee_proto.max_registers, callee_proto.param_count) + 64)
+      Tuple.duplicate(nil, max(callee_proto.max_registers, callee_proto.param_count) + 16)
 
     callee_regs =
       args
@@ -499,7 +499,7 @@ defmodule Lua.VM.Executor do
 
           # Create new register file for the callee
           callee_regs =
-            Tuple.duplicate(nil, max(callee_proto.max_registers, callee_proto.param_count) + 64)
+            Tuple.duplicate(nil, max(callee_proto.max_registers, callee_proto.param_count) + 16)
 
           # Copy arguments into callee registers (params are R[0..N-1])
           callee_regs =
@@ -1104,7 +1104,7 @@ defmodule Lua.VM.Executor do
   # Helper: call a function value inline (used by generic_for)
   defp call_value({:lua_closure, callee_proto, callee_upvalues}, args, _proto, state) do
     callee_regs =
-      Tuple.duplicate(nil, max(callee_proto.max_registers, callee_proto.param_count) + 64)
+      Tuple.duplicate(nil, max(callee_proto.max_registers, callee_proto.param_count) + 16)
 
     callee_regs =
       args