feat: async/await/spawn + fix else-if lowering bug

Self-hosted compiler (buxc2):
- Add async/await/spawn tokens, parsing, HIR, lowering, C emission
- Fix pointer type emission (*T -> T*) in C backend
- Fix sizeof(Type) parsing with parentheses
- Fix import ::{...} infinite loop guard
- Fix int64 emission by avoiding else-if chain workaround
- Add debug-free cli and hir_lower

Bootstrap compiler (Nim):
- Fix critical else-if lowering bug in hir_lower.nim:
  when both elseIfs and else block exist, elseIfs were dropped
  causing all else-if chains to collapse to if+else only

Runtime:
- Fix bux_async_await memory corruption (don't free in run)
- Add bux_remove_from_ready for safe cleanup
- Fix forward declaration and deduplicate bux_now_ms

Docs & examples:
- Update async.bux example with proper int64 params
- Update README, LanguageRef, Stdlib, PLAN for async features
- Mark Phase 7.10 bootstrap loop as completed

All 27 examples pass. buxc2 check/build work on all examples.

examples/async.bux

@@ -5,20 +5,21 @@ extern func bux_async_run();
 extern func bux_async_spawn(fn: *void) -> *void;
 extern func bux_async_await(handle: *void) -> *void;
 extern func bux_async_return(value: *void, size: int64);
+extern func bux_async_sleep(ms: int64);
 
 async func Compute() -> int {
-    PrintLine("Compute: step 1");
-    bux_async_yield();
-    PrintLine("Compute: step 2");
+    PrintLine("Compute: start");
+    bux_async_sleep(100);
+    PrintLine("Compute: after 100ms");
     let result: int = 42;
     bux_async_return((&result) as *void, sizeof(int));
     return result;
 }
 
 async func Double() -> int {
-    PrintLine("Double: step 1");
-    bux_async_yield();
-    PrintLine("Double: step 2");
+    PrintLine("Double: start");
+    bux_async_sleep(50);
+    PrintLine("Double: after 50ms");
     let result: int = 84;
     bux_async_return((&result) as *void, sizeof(int));
     return result;