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/hir_lower.nim

@@ -832,11 +832,11 @@ proc lowerStmt(ctx: var LowerCtx, stmt: Stmt): HirNode =
     let cond = ctx.lowerExpr(stmt.stmtIfCond)
     let thenBlock = ctx.lowerBlock(stmt.stmtIfThen)
     var elseBlock: HirNode = nil
-    if stmt.stmtIfElse != nil:
-      elseBlock = ctx.lowerBlock(stmt.stmtIfElse)
-    elif stmt.stmtIfElseIfs.len > 0:
-      # Desugar else-if chain
+    if stmt.stmtIfElseIfs.len > 0:
+      # Desugar else-if chain, attaching else block if present
       var current: HirNode = nil
+      if stmt.stmtIfElse != nil:
+        current = ctx.lowerBlock(stmt.stmtIfElse)
       for i in countdown(stmt.stmtIfElseIfs.len - 1, 0):
         let elifBranch = stmt.stmtIfElseIfs[i]
         let elifCond = ctx.lowerExpr(elifBranch.cond)
@@ -844,6 +844,8 @@ proc lowerStmt(ctx: var LowerCtx, stmt: Stmt): HirNode =
         current = HirNode(kind: hIf, ifCond: elifCond, ifThen: elifBlock,
                          ifElse: current, typ: makeVoid(), loc: elifBranch.loc)
       elseBlock = current
+    elif stmt.stmtIfElse != nil:
+      elseBlock = ctx.lowerBlock(stmt.stmtIfElse)
     return ctx.flushPending(HirNode(kind: hIf, ifCond: cond, ifThen: thenBlock, ifElse: elseBlock,
                    typ: makeVoid(), loc: loc))