fix(dfir_lang): Add FusedStream requirement to source_stream

[Copilot]

source_stream converts Pending to Ready(None) when polling (line 76), effectively re-polling after termination. This violates the Stream contract unless the stream implements FusedStream, which guarantees Ready(None) is stable.

Changes

• Added FusedStream trait bound to source_stream codegen in check_stream helper
• Updated stream utilities to return Fuse<T> wrappers:
  • unbounded_channel(), unsync_channel()
  • iter_batches_stream()
  • TCP/UDP framing utilities (tcp_framed, udp_framed, etc.)
  • new_inbox() in simulation module
• Updated dependent operators to fuse streams before passing to source_stream:
  • source_interval, source_stdin, source_file
• Added compile-fail test surface_source_stream_not_fused.rs demonstrating the error when using non-fused streams

Example

Before this change, the following would compile but could panic at runtime:

// NotFusedStream panics if polled after returning None
let stream = NotFusedStream::new();
source_stream(stream) -> for_each(drop);  // Would compile, unsafe behavior

After this change, it correctly fails compilation with a clear error about the missing FusedStream bound.

Original prompt

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This section details on the original issue you should resolve

<issue_title>bug(dfir_lang): source_stream should require FusedStream but doesn't</issue_title>
<issue_description>Since we re-poll after Ready(None)</issue_description>

<agent_instructions>You will simply need to update the trait bounds in the codegen in /home/ec2-user/hydro/dfir_lang/src/graph/ops/source_stream.rs

Please add one failing test into dfir_rs/tests/compile-fail (and symlink to file to dfir_rs/tests/compile-fail-stable to test on stable as well)</agent_instructions>

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

• Fixes bug(dfir_lang): source_stream should require FusedStream but doesn't #2419

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[MingweiSamuel]

Suggested change

	fn check_stream<Stream: #root::futures::stream::Stream<Item = Item> + #root::futures::stream::FusedStream + ::std::marker::Unpin, Item>(stream: Stream)
	-> impl #root::futures::stream::Stream<Item = Item> + #root::futures::stream::FusedStream + ::std::marker::Unpin
	fn check_stream<Stream: #root::futures::stream::FusedStream<Item = Item> + ::std::marker::Unpin, Item>(stream: Stream)
	-> impl #root::futures::stream::FusedStream<Item = Item> + ::std::marker::Unpin