panic in sync-lifted `wasi:http/handler` due to `FutureWriter::drop` outside task context

Attempting to write a wasi-http handler with a sync-lifted handler panics in guest code.

## How to Reproduce

Using `-export:` to sync-lift a `wasi:http/handler#handle` export:

```rust
// async: ["-export:wasi:http/handler@0.3.0-rc-2026-03-15#handle"]
fn handle(_req: Request) -> Result {
let headers = Fields::new();
let (tw, tr) = wit_future::new(|| Ok(None::));
drop(tw); // <-- panics
let (response, _) = Response::new(headers, None, tr);
Ok(response)
}
```

Results in a trap/panic in guest code.

## Backtrace

```
thread '' (1) panicked at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/waitable.rs:193:13:
assertion failed: !task.is_null()
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
worker error: error while executing at wasm backtrace:
// ... panic/abort frames ....
12: 0x4dd6 - wit_bindgen::rt::async_support::waitable::WaitableOperation::register_waker::hde47cb997528dce8
at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/waitable.rs:193:13
- wit_bindgen::rt::async_support::waitable::WaitableOperation::poll_complete_with_code::hd22a90f526e6d1de
at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/waitable.rs:338:18
13: 0x57a5 - wit_bindgen::rt::async_support::waitable::WaitableOperation::poll_complete::h605a2b23cc93fc87
at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/waitable.rs:293:14
14: 0x5c81 - as core::future::future::Future>::poll::hc72fcaed7ea6645c
at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/future_support.rs:716:14
- ::write_and_forget::DeferredWrite as alloc::task::Wake>::wake::he1249cadca69f382
at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/future_support.rs:554:62
15: 0x5a79 - wit_bindgen::rt::async_support::future_support::RawFutureWriter::write_and_forget::h235167da4f91e4a1
at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/future_support.rs:506:10
- as core::ops::drop::Drop>::drop::hcaaad8d86644138a
at /Users/paul.osborne/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.55.0/src/rt/async_support/future_support.rs:379:17
- core::ptr::drop_in_place,wasi_http_concurrency_guest::wasi::http::types::ErrorCode>>>::h606931453b035b43
at /Users/paul.osborne/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:805:1
- core::mem::drop::hf433209343d7ac34
at /Users/paul.osborne/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/mem/mod.rs:975:1
- ::handle::h74ebd70f1b9d8869
at /Users/paul.osborne/Projects/Fastly/compute-p3-examples/wasi-http-concurrency/guest/src/lib.rs:249:9
16: 0x2218 - wasi_http_concurrency_guest::exports::wasi::http::handler::_export_handle_cabi::h30c4ecad9fbc835d
at /Users/paul.osborne/Projects/Fastly/compute-p3-examples/wasi-http-concurrency/guest/src/lib.rs:90:1
17: 0x62a2 - wasi:http/handler@0.3.0-rc-2026-03-15#handle
at /Users/paul.osborne/Projects/Fastly/compute-p3-examples/wasi-http-concurrency/guest/src/lib.rs:90:1
```

## Notes

- The `-export:` prefix generates correct ABI (`fn handle`, bare export symbol without `[async-lift]`). wasmtime 44 finds and calls the export; implicit backpressure via `Task.needs_exclusive()` works correctly.
- The panic path goes through `FutureWriter::drop` → `write_and_forget` → `register_waker`, which requires an active task context absent in a sync-lifted call. See [#1456](https://github.com/bytecodealliance/wit-bindgen/issues/1456) for discussion.
- Related upstream discussion: [WebAssembly/WASI #898](https://github.com/WebAssembly/WASI/issues/898) (instance reuse opt-out via sync-lift).

## Versions

wit-bindgen 0.55.0, wasmtime 44.0.1, wasi:http 0.3.0-rc-2026-03-15