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.

src_bux/token.bux

@@ -133,6 +133,11 @@ const tkNewLine: int = 99;
 const tkEndOfFile: int = 100;
 const tkUnknown: int = 101;
 
+// Async / concurrency
+const tkAsync: int = 102;
+const tkAwait: int = 103;
+const tkSpawn: int = 104;
+
 // ---------------------------------------------------------------------------
 // Token struct
 // ---------------------------------------------------------------------------
@@ -154,6 +159,8 @@ func Token_IsKeyword(kind: int) -> bool {
     if kind >= tkBreak && kind <= tkMatch { return true; }
     if kind >= tkFunc && kind <= tkExtern { return true; }
     if kind >= tkAs && kind <= tkSuper { return true; }
+    if kind == tkSizeOf { return true; }
+    if kind >= tkAsync && kind <= tkSpawn { return true; }
     return false;
 }