feat: add wasm-gc support for Promise bridging, ReadableStream, and HTTP client

[duobei]

Summary

Add wasm-gc implementations for js_async and http packages, building on the timer/event-loop foundation from #314.

• Promise bridging (js_async): Promise::wait(), run_promise(), Promise::from_async() via Deferred pattern (wasm-gc cannot directly call JS functions received as parameters)
• ReadableStream (js_async): callback-based byte copying for Bytes/Uint8Array boundary crossing (one FFI call per chunk)
• HTTP client (http): fetch-based client with RawExternRef + %identity for cross-package #external type limitation in WASM import signatures

New files

File	Lines	Description
src/js_async/js_async.wasm-gc.mbt	217	Promise + AbortController + JsError
src/js_async/readable_stream.wasm-gc.mbt	180	ReadableStream wrapper + JsReadableStream pipe
src/http/client.wasm-gc.mbt	299	Fetch-based HTTP client
src/js_async/wasm-gc-imports.js	53	JS imports: moonbitlang_async_js (11) + moonbitlang_async_stream (11)
src/http/wasm-gc-imports.js	27	JS imports: moonbitlang_async_http (8)

Key design decisions

• Deferred pattern replaces new Promise(executor) — wasm-gc closures (funcref) can be called from JS, but JS functions received as parameters cannot be called from MoonBit
• Callback-based Bytes copying — Bytes on wasm-gc is a GC type (not Uint8Array), so JS iterates the Uint8Array and calls a MoonBit set_byte callback; reverse direction uses a get_byte callback
• RawExternRef + %identity — wasm-gc compiler requires WASM import parameter/return types to be defined in the same compilation unit; cross-package #external types are wrapped via local externref aliases
• JsHeaders::for_each replaces to_array — wasm-gc Array is not a JS Array

Verification

• moon check --target wasm-gc --deny-warn ✅
• moon check --target native --deny-warn ✅
• moon check --target js --deny-warn ✅
• moon test --target native — 407/415 passed (8 pre-existing IPv6/WebSocket env failures)

Depends on

• feat: add wasm-gc support for timer, event loop, and time #314 (wasm-gc timer, event loop, time)

[Copilot]

Pull request overview

Adds wasm-gc target support across async JS interop and HTTP by introducing Promise/AbortController bridging, ReadableStream wrappers for byte transfer, and a fetch-based HTTP client wired via WASM imports (building on the wasm-gc timer/event-loop foundation).

Changes:

• Add wasm-gc implementations for @js_async Promise bridging and ReadableStream interop, plus reference JS import objects.
• Add a wasm-gc fetch-based @http.Client implementation and its reference JS import object.
• Extend wasm-gc runtime plumbing (event loop + timer + wall-clock time) and update package target mappings.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/js_async/wasm-gc-imports.js	Reference host import object for wasm-gc Promise + stream bridging.
src/js_async/readable_stream.wasm-gc.mbt	wasm-gc ReadableStream wrapper + pipe-to-stream interop via callback byte copying.
src/js_async/moon.pkg	Enables wasm-gc targets for new js_async implementations; narrows unimplemented targets.
src/js_async/js_async.wasm-gc.mbt	wasm-gc Promise/AbortController/JsError bridging via Deferred pattern and WASM imports.
src/internal/time/time.mbt	Implements ms_since_epoch() for wasm-gc via Date.now() WASM import.
src/internal/event_loop/wasm-gc-imports.js	Reference host import object for wasm-gc timer + time imports.
src/internal/event_loop/timer.wasm-gc.mbt	wasm-gc timer implementation using setTimeout/clearTimeout WASM imports.
src/internal/event_loop/moon.pkg	Registers wasm-gc event loop + timer targets; adjusts unimplemented targets.
src/internal/event_loop/event_loop.wasm-gc.mbt	wasm-gc cooperative rescheduling implementation.
src/http/wasm-gc-imports.js	Reference host import object for wasm-gc fetch client and Headers/Response helpers.
src/http/moon.pkg	Enables wasm-gc target for new HTTP client and for request model package file.
src/http/client.wasm-gc.mbt	wasm-gc fetch-based HTTP client using RawExternRef + %identity to bridge externref signatures.

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