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/lexer.bux

@@ -294,6 +294,9 @@ func lexKeywordKind(text: String) -> int {
     if String_Eq(text, "self") { return tkSelf; }
     if String_Eq(text, "super") { return tkSuper; }
     if String_Eq(text, "sizeof") { return tkSizeOf; }
+    if String_Eq(text, "async") { return tkAsync; }
+    if String_Eq(text, "await") { return tkAwait; }
+    if String_Eq(text, "spawn") { return tkSpawn; }
     return tkIdent;
 }