feat: Phase 8.3 Concurrency — tasks, channels, spawn

- Add async/await/spawn keywords to lexer
- C runtime: pthread wrappers (bux_task_spawn, bux_task_join, bux_task_sleep)
- C runtime: mutex/condvar channel implementation (bux_channel_new/send/recv/close/free)
- Build command adds -pthread flag for C compilation
- Std::Task module: TaskHandle, Task_Spawn, Task_Join, Task_Sleep
- Std::Channel module: generic Channel<T> with Send, Recv, Close, Free
- Parser: spawn Expr() expression
- Sema: ekSpawn returns *void
- HIR: hSpawn node lowered from ekSpawn
- C backend: emit bux_task_spawn(FuncName, arg) for spawn expressions
- Example: examples/concurrency.bux (thread + channel demo)
- Example: examples_pkg/concurrency working project

src/parser.nim

@@ -444,6 +444,18 @@ proc parsePrimary(p: var Parser): Expr =
     let ty = p.parseType()
     discard p.expect(tkRParen, "expected ')'")
     return Expr(kind: ekSizeOf, loc: loc, exprSizeOfType: ty)
+  of tkSpawn:
+    discard p.advance()  # spawn
+    let callee = p.parsePrimary()
+    var args: seq[Expr] = @[]
+    if p.check(tkLParen):
+      discard p.advance()  # (
+      while not p.check(tkRParen) and not p.isAtEnd:
+        args.add(p.parseExpr())
+        if p.check(tkComma):
+          discard p.advance()
+      discard p.expect(tkRParen, "expected ')' after spawn arguments")
+    return Expr(kind: ekSpawn, loc: loc, exprSpawnCallee: callee, exprSpawnArgs: args)
   of tkHashLine:
     discard p.advance()
     return Expr(kind: ekIntrinsic, loc: loc, exprIntrinsic: ikLine)