fix(jsruntime/http): V8 listen keepalive + express handler smoke

[proggeramlug]

Summary

Three bug fixes that unblock the express end-to-end smoke against the V8-fallback http.createServer bridge from #994:

• V8 bind keepalive: JsRuntimeState::last_poll_was_pending is set by poll_v8_event_loop_once whenever deno_core reports refed ops in flight, and jsruntime_has_active_handles returns 1 while it's set. Pre-fix, app.listen(port, cb) returned to the codegen-emitted outer event loop while op_perry_http_listen's TcpListener::bind future was still suspended on the multi-thread runtime, the outer loop saw no active source, and the program exited before bind ever resolved.
• Express handler polyfills: added setImmediate / clearImmediate globals (express's router uses them to break middleware recursion) and Buffer.allocUnsafeSlow (so safe-buffer, used by express/lib/response.js, doesn't fall through to a for...in copyProps that silently drops non-enumerable static methods like Buffer.isBuffer). Without these, every (_req, res) => res.send('pong') threw and the http shim returned 500.
• Cb ordering + async op_perry_fetch: Server.listen now schedules the listening callback via Promise.resolve().then(() => cb()) so the JS accept loop registers its first op_perry_http_accept(sid) op before the trampoline-driven callback gets a chance to block the V8 thread. op_perry_fetch is now an async op wrapping the blocking ureq call in tokio::task::spawn_blocking, with the polyfill updated to await core.ops.op_perry_fetch(...).

External clients (curl http://127.0.0.1:port/...) hit a Perry-compiled express server and get the route body. The self-fetch case (async () => { await fetch('http://127.0.0.1:port/...') } inside the listen-cb) still deadlocks because Perry's native async lowering busy-waits on the V8 thread inside trampolines, blocking the IIFE's accept loop continuation — documented in CHANGELOG as a deferred issue gated on real Future-style state-machine lowering.

Verification

• New regression test-files/test_issue_997_v8_listen_keepalive.ts: synchronous listen-cb that closes the server immediately. Pre-fix hangs forever under timeout; post-fix prints listening and exits 0.
• Existing test-files/test_http_createserver_v8.ts still passes (200 hello + exit 0).
• Express smoke (app.get('/ping', (_req, res) => res.send('pong'))) returns pong via external curl. Confirmed locally.
• Fastify end-to-end smoke ({"msg":"pong"}) still works through perry-stdlib's bundled fastify path.

Test plan

• CI lint passes
• CI cargo-test passes
• CI parity passes
• CI compile-smoke passes
• Local: target/release/perry test-files/test_issue_997_v8_listen_keepalive.ts -o /tmp/t && timeout 5 /tmp/t prints listening and exits 0
• Local: target/release/perry test-files/test_http_createserver_v8.ts -o /tmp/t2 && timeout 10 /tmp/t2 prints 200 hello