feat(hydro_lang): add flatten_stream, flat_map_stream to Stream

[MingweiSamuel]

STACK PREV: #2688

• Added FlatMapStream variant to HydroNode enum in compile/ir/mod.rs with
  the same shape as FlatMap (f, input, metadata).
• Handled FlatMapStream in all match arms: traversal, deep_clone, lowering
  (emits flat_map_stream(#f)), expression refs, metadata/metadata_mut,
  input(), and print_root().
• Updated viz/render.rs to handle FlatMapStream in the transform group.
• Added flat_map_stream() and flatten_stream() methods to Stream in
  hydro_lang/src/live_collections/stream/mod.rs. flat_map_stream takes a
  closure mapping items to futures::Stream; flatten_stream flattens items
  that are already Streams.

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[Copilot]

Pull request overview

Adds first-class support for flattening/flat-mapping async streams in Hydro/DFIR by introducing a new IR node and surfacing the corresponding operators/methods across the stack.

Changes:

• Introduces HydroNode::FlatMapStream and threads it through IR traversal/cloning/lowering/metadata/printing.
• Adds DFIR surface operators flat_map_stream and flatten_stream (pull + push codegen paths).
• Adds Stream::{flat_map_stream, flatten_stream} in hydro_lang and adds DFIR surface tests covering basic + empty cases.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
hydro_lang/src/viz/render.rs	Renders FlatMapStream in the “single-expr transform” group for visualization.
hydro_lang/src/live_collections/stream/mod.rs	Adds flat_map_stream / flatten_stream Stream APIs that build HydroNode::FlatMapStream.
hydro_lang/src/compile/ir/mod.rs	Adds the FlatMapStream IR variant and lowers it to flat_map_stream(#f) in emitted DFIR.
dfir_rs/tests/surface_flat_stream.rs	Adds surface-level tests for flat_map_stream and flatten_stream.
dfir_lang/src/graph/ops/mod.rs	Registers the new operators so they’re available to DFIR syntax/codegen.
dfir_lang/src/graph/ops/flatten_stream.rs	Defines flatten_stream operator constraints and codegen for pull/push.
dfir_lang/src/graph/ops/flat_map_stream.rs	Defines flat_map_stream operator constraints and codegen for pull/push.

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