mruby-task: add gc protection and optimize task operations

Implements dual-mechanism GC protection and optimizes task lookup
using pointer arithmetic based on PicoRuby reference implementation.

GC Protection:
- Add mrb_gc_register/unregister to protect Task objects
- Implement mrb_task_mark_all() to mark task contexts during GC
- Store proc reference in mrb_task to prevent premature collection
- Integrate marking into gc.c root_scan_phase

Performance Optimizations:
- Add MRB2TASK macro for O(1) context-to-task conversion
- Optimize Task.current: O(n) queue search -> O(1) pointer arithmetic
- Optimize Task.pass: simplify to root context check
- Optimize Task.join: use MRB2TASK for current task lookup

Bug Fixes:
- Fix MRB_TASK_CREATED/STOPPED to use MRB_FIBER_TERMINATED
- Add safety check to prevent execution of terminated tasks
- Initialize callinfo PC to bytecode start in task_init_context

Co-authored-by: Claude <noreply@anthropic.com>

include/mruby/internal.h

@@ -266,4 +266,9 @@ mrb_value mrb_bint_lcm(mrb_state *mrb, mrb_value x, mrb_value y);
 mrb_value mrb_bint_abs(mrb_state *mrb, mrb_value x);
 #endif
 
+#ifdef MRB_USE_TASK_SCHEDULER
+/* GC marking for task scheduler */
+void mrb_task_mark_all(mrb_state *mrb);
+#endif
+
 #endif  /* MRUBY_INTERNAL_H */