feat: add benchmark runner with fib implementations and OS.clock primitive

Add a benchmark runner script (benchmark/run_all) that executes multiple
fibonacci implementations across languages (Lua, Python, Ruby, Wren) and
computes mean, median, and standard deviation from 7 runs. Update existing
fib benchmarks to loop 5 times at fib(30) with elapsed timing. Introduce
OS.clock primitive in C core and OS class to support timing in Wren.
Adjust number formatting to "%.14g" for consistent output precision.

benchmark/fib.wren

@@ -6,4 +6,10 @@ var fib = fn(n) {
   }
 }
 
-io.write(fib.call(35))
+var start = OS.clock
+var i = 0
+while (i < 5) {
+  io.write(fib.call(30))
+  i = i + 1
+}
+io.write("elapsed: " + (OS.clock - start).toString)