Redesign stream as union and add streams.Enumerate (#908)

[emil14]

Context

Implements core migration for #908 in scope of tracker #1033.

What is done

• migrate stream<T> from struct to tagged union (Open | Data T | Close)
• add parser/analyzer support for generic union literals in sender positions
• migrate runtime stream funcs to Open/Data/Close protocol
• add streams.Indexed<T>, streams.Enumerate<T>, streams.Just<T>
• add runtime stream union helpers (IsData/IsClose/UnwrapData)
• stabilize streams.ForEach via runtime controller (stream_for_each_controller) to preserve ordering and backpressure
• fix streams.Filter event routing under union-stream semantics
• remove implicit non-union->Data coercion in stream helper path

Verification

Passing:

• go test ./internal/runtime/funcs ./examples/filter_list -count=1 -timeout 5m
• go test ./examples/for_loop_over_list -count=1 -timeout 8m
• go test ./examples/filter_list ./examples/for_loop_over_list -count=1 -timeout 9m

Note:

• local golangci-lint binary in this environment is built with older Go (go1.25), so direct golangci-lint run ./... is blocked against target go1.26.0.

Refs #908
Refs #1033