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/ast.bux

@@ -93,6 +93,8 @@ const ekTry: int = 20;
 const ekUnwrap: int = 23;
 const ekBlock: int = 21;
 const ekMatch: int = 22;
+const ekSpawn: int = 24;
+const ekAwait: int = 25;
 
 struct Expr {
     kind: int,
@@ -223,6 +225,7 @@ struct Decl {
     line: uint32,
     column: uint32,
     isPublic: bool,
+    isAsync: bool,
     // Names
     strValue: String,        // decl name
     strValue2: String,       // interface name, dll name, module path