feat: replace fiber run() with transfer() and suspend() for scheduler-friendly semantics

Remove the run() method from fibers and introduce transfer() for non-stack-based fiber switching, along with suspend() to pause the interpreter and return control to the host application. Update Timer.sleep() to use runNextScheduled_() instead of Fiber.yield(), and add Timer.schedule() for creating independently scheduled fibers. Refactor the core fiber runtime to support transfer semantics, including proper error handling for aborted fibers and self-transfer edge cases. Fix a missing quote in test.py's runtime error validation string.

doc/site/core/fiber.markdown

@@ -19,6 +19,14 @@ fiber is run. Does not immediately start running the fiber.
 
 The currently executing fiber.
 
+### Fiber.**suspend**()
+
+Pauses the current fiber, and stops the interpreter. Control returns to the
+host application.
+
+To resume execution, the host application will need to invoke the interpreter
+again. If there is still a reference to the suspended fiber, it can be resumed.
+
 ### Fiber.**yield**()
 
 Pauses the current fiber and transfers control to the parent fiber. "Parent"
@@ -119,10 +127,10 @@ Invokes the fiber or resumes the fiber if it is in a paused state and sets
 Whether the fiber's main function has completed and the fiber can no longer be
 run. This returns `false` if the fiber is currently running or has yielded.
 
-### **run**()
+### **transfer**()
 
 **TODO**
 
-### **run**(value)
+### **transfer**(value)
 
 **TODO**