Add libuv-backed MultiUv for event-driven concurrent HTTP transfers

[Copilot]

The existing Multi interface uses curl_multi_poll — a blocking poll — making it unsuitable for integration with external event loops or high-concurrency workloads. This PR adds MultiUv, which wires libcurl's socket-action API into libuv's epoll/kqueue-based event loop.

New: src/MultiUv.zig

Implements the curl_multi_socket_action integration pattern:

• CURLMOPT_SOCKETFUNCTION → creates/destroys uv_poll_t handles per curl socket
• CURLMOPT_TIMERFUNCTION → drives curl's timeout via a shared uv_timer_t
• CompletionCallback fired per-transfer inside the event loop (no polling loop needed)
• Heap-allocated (create/destroy) to keep libuv handle addresses stable

const loop = c_uv.uv_default_loop();
var multi = try MultiUv.create(allocator, loop, onComplete, null);
defer multi.destroy();

try multi.addHandle(&easy1);
try multi.addHandle(&easy2);

_ = c_uv.uv_run(loop, c_uv.UV_RUN_DEFAULT); // all transfers run here

Build system (build.zig)

New -Dlibuv=true flag (default: false):

• Passes has_libuv into build_info; root.zig exports MultiUv as void when disabled, so uv headers are never required without the flag
• module.linkSystemLibrary("uv", .{}) propagates -luv to all artifacts that import the module
• Adds run-multi-uv step and libuv check/test targets

src/root.zig

pub const MultiUv = if (build_info.has_libuv) @import("MultiUv.zig") else void;

Dead-branch @import means uv.h is never processed unless the flag is set.

examples/multi_uv.zig

Three concurrent GETs via uv_default_loop() + uv_run(), mirroring the existing multi.zig writer pattern.

Makefile

Adds run-libuv target (zig build run-multi-uv -Dlibuv=true).

Warning

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Integrate with libuv for better perf</issue_title>
<issue_description>https://docs.libuv.org/en/v1.x/guide/utilities.html#external-i-o-with-polling</issue_description>

<agent_instructions>调研与libuv集成方案，实现高性能请求访问
</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Integrate with libuv for better perf #10

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