feat: implement NaN tagging for Value union and hoist bytecode/IP into locals in interpret loop

Switch Value from a struct with type/num/obj fields to a NaN-tagged uint64_t/double union, enabling faster type checks and unboxed numbers. Refactor all macros and accessors (IS_OBJ, AS_OBJ, AS_NUM, etc.) to work with the new representation. Hoist CallFrame pointer, instruction pointer, and bytecode array into local variables inside the interpret loop, updating them on frame push/pop. Enable NAN_TAGGING define and DEBUG_GC_STRESS in common.h. Update Xcode project to use c99 standard, pedantic warnings, and treat warnings as errors. Add benchmark/fib.txt with timing comparisons showing fib benchmark within 25% of Lua.

benchmark/fib.txt

@@ -0,0 +1,37 @@
+Value is just Obj* and all values are boxed.
+→ time ../build/Release/wren fib.wren
+9.22746e+06
+
+real  0m12.688s
+user  0m12.076s
+sys 0m0.601s
+
+---
+
+Unboxed singletons and numbers. Value is a struct of ValueType, double, Obj* obj.
+→ time ../build/Release/wren fib.wren
+9.22746e+06
+
+real  0m3.233s
+user  0m3.229s
+sys 0m0.003s
+
+---
+
+NaN tagged values.
+→ time ../build/Release/wren fib.wren
+9.22746e+06
+
+real  0m3.100s
+user  0m3.097s
+sys 0m0.002s
+
+---
+
+Hoist bytecode and IP into locals in interpret loop.
+→ time ../build/Release/wren fib.wren
+9.22746e+06
+
+real  0m2.490s
+user  0m2.487s
+sys 0m0.002s