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.

docs/LanguageRef.md

@@ -18,7 +18,8 @@ This document describes the Bux programming language as implemented by the boots
 10. [Generics](#generics)
 11. [Error Handling](#error-handling)
 12. [Modules and Imports](#modules-and-imports)
-13. [Operators](#operators)
+13. [Async/Await](#asyncawait)
+14. [Operators](#operators)
 
 ---
 
@@ -42,6 +43,7 @@ Identifiers start with a letter or underscore, followed by letters, digits, or u
 func, let, var, const, type, struct, enum, union, interface, extend
 module, import, pub, extern, if, else, while, do, loop, for, in
 break, continue, return, match, as, is, null, self, super, sizeof
+async, await, spawn
 ```
 
 ### String Literals
@@ -499,6 +501,66 @@ func PrivateFunc() -> int {
 
 ---
 
+## Async/Await
+
+Bux supports stackful coroutines via `async`/`await` with a round-robin scheduler.
+
+### Declaring Async Functions
+
+```bux
+async func Compute() -> int {
+    PrintLine("step 1");
+    bux_async_yield();
+    PrintLine("step 2");
+    return 42;
+}
+```
+
+### Spawning Tasks
+
+```bux
+let handle = spawn Compute();
+```
+
+### Awaiting Results
+
+```bux
+let result: int = handle.await as int;
+```
+
+### Full Example
+
+```bux
+import Std::Io::{PrintLine, PrintInt};
+
+async func Compute() -> int {
+    PrintLine("Compute: start");
+    bux_async_yield();
+    PrintLine("Compute: done");
+    return 42;
+}
+
+func Main() -> int {
+    let h = spawn Compute();
+    let r: int = h.await as int;
+    PrintInt(r);
+    return 0;
+}
+```
+
+### Runtime Functions
+
+| Function | Description |
+|----------|-------------|
+| `bux_async_yield()` | Yield control to the scheduler |
+| `bux_async_spawn(fn)` | Create a new coroutine from a function |
+| `bux_async_await(handle)` | Block until coroutine completes, return result |
+| `bux_async_run()` | Run the scheduler (called implicitly from main) |
+| `bux_async_sleep(ms)` | Sleep for `ms` milliseconds (non-blocking) |
+| `bux_async_return(value, size)` | Copy return value into task result buffer |
+
+---
+
 ## Operators
 
 ### Arithmetic