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/hir_lower.bux

@@ -122,6 +122,25 @@ func Lcx_LowerExpr(ctx: *LowerCtx, expr: *Expr) -> *HirNode {
         return n;
     }
 
+    // spawn Callee(args)
+    if kind == ekSpawn {
+        n.kind = hSpawn;
+        if expr.child1 != null as *Expr && expr.child1.kind == ekIdent {
+            n.strValue = expr.child1.strValue;
+        }
+        if expr.child2 != null as *Expr {
+            n.child1 = Lcx_LowerExpr(ctx, expr.child2);
+        }
+        return n;
+    }
+
+    // expr.await
+    if kind == ekAwait {
+        n.kind = hAwait;
+        n.child1 = Lcx_LowerExpr(ctx, expr.child1);
+        return n;
+    }
+
     // Index: arr[idx]
     if kind == ekIndex {
         n.kind = hIndexPtr;