feat: add input validation and stack trace filtering for Timer.sleep

Add type and range checks to Timer.sleep() in both C source and Wren
implementation, aborting with descriptive errors for non-numeric or
negative arguments. Introduce six new test files covering sleep with
fibers, float durations, main fiber, zero milliseconds, negative
values, and non-numeric input. Fix stack trace parsing in test runner
to skip builtin library frames by iterating through error lines until
a main-line match is found.

src/module/timer.c

@@ -11,6 +11,8 @@
 static const char* timerLibSource =
 "class Timer {\n"
 "  static sleep(milliseconds) {\n"
+"    if (!(milliseconds is Num)) Fiber.abort(\"Milliseconds must be a number.\")\n"
+"    if (milliseconds < 0) Fiber.abort(\"Milliseconds cannot be negative.\")\n"
 "    startTimer_(milliseconds, Fiber.current)\n"
 "    Fiber.yield()\n"
 "  }\n"

src/module/timer.h

@@ -3,7 +3,6 @@
 
 #include "wren.h"
 
-// TODO: Coherent naming scheme.
 char* timerGetSource();
 
 WrenForeignMethodFn timerBindForeign(

src/module/timer.wren

@@ -1,5 +1,7 @@
 class Timer {
   static sleep(milliseconds) {
+    if (!(milliseconds is Num)) Fiber.abort("Milliseconds must be a number.")
+    if (milliseconds < 0) Fiber.abort("Milliseconds cannot be negative.")
     startTimer_(milliseconds, Fiber.current)
     Fiber.yield()
   }