Remove Copy trait from BuildEvent and PackageTarballInfo

[Copilot]

The Copy trait was implemented on BuildEvent<'a> and PackageTarballInfo<'a> despite these being event types containing borrowed references. While technically valid (all fields are Copy), this is semantically incorrect—events represent discrete state changes and shouldn't be trivially duplicated.

Changes

• Removed Copy from event types: BuildEvent<'a> and PackageTarballInfo<'a> now derive only Clone
• Made duplication explicit: Updated PipelineReceiver::on_event to explicitly clone the event when forwarding to inner receiver

// Before: implicit copy due to Copy trait
self.inner.on_event(event);
match event { ... }

// After: explicit clone makes cost visible
self.inner.on_event(event.clone());
match event { ... }

This follows Rust idioms where Clone without Copy signals that duplication has semantic weight, appropriate for event types.

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[elrrrrrrr]

Copilot 这个说法有点教条了。它的逻辑是："event 代表一次性的状态变更，不应该被随意复制"。

但这里的 BuildEvent<'a> 并不拥有任何数据——它全是借用引用（&'a str、&'a Path），本质上就是一个只读视图。Copy 它不会复制任何实际状态，只是多了一个指向相同数据的指针。这跟复制一个拥有
owned data 的事件（比如 Event { data: String }) 完全不同。

如果是 BuildEvent 持有 String、PathBuf 这些 owned 类型，那 Copilot 说的有道理——随意 Copy/Clone 会有性能和语义问题。但对一个全是引用的轻量 enum，Copy 就是最自然的选择。

实际上你也可以问 Copilot：不用 Copy 的话怎么解决 PipelineReceiver::on_event 里 event 被用两次的问题？它只能建议你 .clone()——对一个全是引用的类型手动 clone，反而更别扭。

@xusd320 😄 他们打起来了