feat: Phase 8.3 async/await syntax + runtime

- AST: declFuncIsAsync flag, ekAwait expression kind
- Parser: async func declarations, expr.await postfix operator
- Sema: allow await anywhere (blocks current thread until task completes)
- HIR: ekAwait lowered to bux_task_join(handle) call
- C backend: async func emitted as regular function
- Example: examples/async.bux + examples_pkg/async project
- All 20 existing examples pass, selfhost build works

src/ast.nim

@@ -118,6 +118,7 @@ type
     ekTry
     ekUnwrap        ## expr! — unwrap or panic
     ekSpawn         ## spawn expr — create a new task
+    ekAwait         ## expr.await — suspend until future resolves
     ekBlock
     ekMatch
 
@@ -200,6 +201,8 @@ type
     of ekSpawn:
       exprSpawnCallee*: Expr
       exprSpawnArgs*: seq[Expr]
+    of ekAwait:
+      exprAwaitOperand*: Expr
     of ekBlock:
       exprBlock*: Block
     of ekMatch:
@@ -335,6 +338,7 @@ type
       declFuncAsm*: bool
       declFuncCallConv*: CallingConvention
       declFuncConst*: bool          ## const func — evaluable at compile time
+      declFuncIsAsync*: bool        ## async func — returns Future<T>
       declFuncName*: string
       declFuncTypeParams*: seq[TypeParam]
       declFuncParams*: seq[Param]

src/hir_lower.nim

@@ -768,6 +768,10 @@ proc lowerExpr(ctx: var LowerCtx, expr: Expr): HirNode =
     return HirNode(kind: hSpawn, spawnCallee: calleeName, spawnArgs: args,
                    typ: makePointer(makeVoid()), loc: loc)
 
+  of ekAwait:
+    let lowered = ctx.lowerExpr(expr.exprAwaitOperand)
+    return hirCall("bux_task_join", @[lowered], makeVoid(), loc)
+
   else:
     return HirNode(kind: hLit, litToken: Token(kind: tkIntLiteral, text: "0", loc: loc),
                    typ: makeVoid(), loc: loc)

src/parser.nim

@@ -527,10 +527,14 @@ proc parsePostfix(p: var Parser): Expr =
       discard p.expect(tkRBracket, "expected ']' to close index")
       left = Expr(kind: ekIndex, loc: loc, exprIndexObj: left, exprIndexIdx: idx)
     of tkDot:
-      # Field expression
+      # Field expression or .await
       discard p.advance()
-      let fieldName = p.expectIdentOrKeyword("expected field name after '.'").text
-      left = Expr(kind: ekField, loc: loc, exprFieldObj: left, exprFieldName: fieldName)
+      if p.check(tkAwait):
+        discard p.advance()
+        left = Expr(kind: ekAwait, loc: loc, exprAwaitOperand: left)
+      else:
+        let fieldName = p.expectIdentOrKeyword("expected field name after '.'").text
+        left = Expr(kind: ekField, loc: loc, exprFieldObj: left, exprFieldName: fieldName)
     of tkPlusPlus, tkMinusMinus:
       let op = p.advance().kind
       left = Expr(kind: ekPostfix, loc: loc, exprPostfixOp: op, exprPostfixOperand: left)
@@ -971,7 +975,7 @@ proc parseParamList(p: var Parser, allowVariadic: bool = false): seq[Param] =
       discard p.advance()
   discard p.expect(tkRParen, "expected ')' to close parameter list")
 
-proc parseFuncDecl(p: var Parser, isPublic: bool, isAsm: bool, attrs: ParsedAttrs, isConst: bool = false): Decl =
+proc parseFuncDecl(p: var Parser, isPublic: bool, isAsm: bool, attrs: ParsedAttrs, isConst: bool = false, isAsync: bool = false): Decl =
   let loc = p.currentLoc
   discard p.expect(tkFunc, "expected 'func'")
   let name = p.expect(tkIdent, "expected function name").text
@@ -991,7 +995,7 @@ proc parseFuncDecl(p: var Parser, isPublic: bool, isAsm: bool, attrs: ParsedAttr
   return Decl(kind: dkFunc, loc: loc, isPublic: isPublic,
               declAttrs: declAttrs,
               declFuncAsm: isAsm, declFuncCallConv: attrs.callConv,
-              declFuncConst: isConst,
+              declFuncConst: isConst, declFuncIsAsync: isAsync,
               declFuncName: name, declFuncTypeParams: typeParams,
               declFuncParams: params, declFuncReturnType: retType,
               declFuncBody: body)
@@ -1278,10 +1282,15 @@ proc parseDecl(p: var Parser): Decl =
   if p.check(tkConst) and p.peek(1) == tkFunc:
     isConst = true
     discard p.advance()
+
+  var isAsync = false
+  if p.check(tkAsync) and p.peek(1) == tkFunc:
+    isAsync = true
+    discard p.advance()
   
   case p.peek()
   of tkFunc:
-    return p.parseFuncDecl(isPublic, false, attrs, isConst)
+    return p.parseFuncDecl(isPublic, false, attrs, isConst, isAsync)
   of tkStruct:
     return p.parseStructDecl(isPublic)
   of tkEnum:

src/sema.nim

@@ -42,6 +42,7 @@ type
     interfaceTable*: Table[string, Decl]
     # Borrow checker state
     checkedFunc*: bool  ## true inside @[Checked] function
+    currentFuncIsAsync*: bool  ## true inside async func
 
 # ---------------------------------------------------------------------------
 # Helpers
@@ -1007,6 +1008,10 @@ proc checkExpr(sema: var Sema, expr: Expr, scope: Scope): Type =
     for arg in expr.exprSpawnArgs:
       discard sema.checkExpr(arg, scope)
     return makePointer(makeVoid())
+  of ekAwait:
+    let operand = sema.checkExpr(expr.exprAwaitOperand, scope)
+    # await on a task handle returns void for now
+    return makeVoid()
   of ekSpread:
     return sema.checkExpr(expr.exprSpreadOperand, scope)
 
@@ -1100,7 +1105,9 @@ proc checkFunc(sema: var Sema, decl: Decl) =
   if decl.declFuncTypeParams.len > 0:
     return
   let wasChecked = sema.checkedFunc
+  let wasAsync = sema.currentFuncIsAsync
   sema.checkedFunc = "Checked" in decl.declAttrs
+  sema.currentFuncIsAsync = decl.declFuncIsAsync
   var funcScope = newScope(sema.globalScope)
   # Add type parameters to type table for resolution
   var addedTypeParams: seq[string] = @[]
@@ -1119,6 +1126,7 @@ proc checkFunc(sema: var Sema, decl: Decl) =
   for tp in addedTypeParams:
     sema.typeTable.del(tp)
   sema.checkedFunc = wasChecked
+  sema.currentFuncIsAsync = wasAsync
 
 # ---------------------------------------------------------------------------
 # Second pass: check all function bodies