squash! update DESIGN.md

DESIGN.md

@@ -54,9 +54,9 @@ they are almost universally:
 deliberate changes & known bugs
 ===============================
 
-- Forking is no longer supported. `fork(2)` and threads don't go well together.
-  After the fork, only the calling thread remains; the V8 thread is gone and
-  likely left mutexes and other resources in an undefined state.
+- `MiniRacer::Platform.set_flags! :single_threaded` still runs everything on
+  the same thread but is prone to crashes in Ruby < 3.4.0 due to a Ruby runtime
+  bug that clobbers thread-local variables.
 
 - The `Isolate` class is gone. Maintaining a one-to-many relationship between
   isolates and contexts in a multi-threaded environment had a bad cost/benefit
@@ -66,6 +66,8 @@ deliberate changes & known bugs
 - The `marshal_stack_depth` argument is still accepted but ignored; it's no
   longer necessary.
 
+- The `ensure_gc_after_idle` argument is a no-op in `:single_threaded` mode.
+
 - The `timeout` argument no longer interrupts long-running Ruby code. Killing
   or interrupting a Ruby thread executing arbitrary code is fraught with peril.