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.

test/timer/sleep_float.wren

@@ -0,0 +1,20 @@
+import "timer" for Timer
+
+// These are both rounded to 1, so "a" should complete first.
+Fiber.new {
+  Timer.sleep(1.5)
+  IO.print("a")
+}.call()
+
+Fiber.new {
+  Timer.sleep(1.3)
+  IO.print("b")
+}.call()
+
+IO.print("main")
+Timer.sleep(3)
+IO.print("done")
+// expect: main
+// expect: a
+// expect: b
+// expect: done