[rush reporter][R5] Ship the feature-flagged operation stream demo

Start by tracing the named OperationGraph.eventSink and OperationExecutionRecord hooks into OperationStreamEmitter, then compare the existing StreamCollator path and built-in reporters. Use the direct rush build entry point and the one-command demo fixture to verify legacy and opt-in behavior. Done means parity when disabled, complete event coverage without duplicate output, and the documented reporter demonstrations pass.

Parent: #5858

Blocked by: #5975, #5976

Goal

Deliver the first end-to-end, demoable Rush reporter path for direct rush build invocations, fully reversible behind opt-in controls.

Approach

Reuse the existing OperationGraph.eventSink/OperationExecutionRecord hooks to adapt scheduler activity into OperationStreamEmitter semantic events. Do not rewrite scheduling. While the opt-in is disabled, retain the exact StreamCollator path. While enabled, keep any collator plumbing needed internally but route its destination to a sink that cannot produce duplicate terminal output; the selected reporters become the sole visible presentation owners. Remove the primary-path collator only after parity is proven.

Scope

• Emit phase-aware operation registration, status transitions, raw stdout/stderr chunks, stream close, completion, and aggregate command-result events.
• Wire the built-in default, plaintext, ai, json, file, and legacy reporters for the direct frontend path.
• Demonstrate the compact default reporter (spinner/activity/result) and always-on full-detail file.
• Preserve detailed grouping in plaintext/file reporters and keep problem-matcher input uncollated.
• Cover success, cache hit, warning, failure, blocked operation, cancellation, and watch iteration behavior.

Acceptance criteria

• Flag off: representative legacy golden output is byte-for-byte unchanged.
• Flag on: reporters are the only visible writers; no duplicated or lost operation chunks.
• The default interactive demo stays within the intended compact activity/result presentation and points to the full log.
• Plaintext/file output reconstructs operation grouping from raw events.
• AI output is remediation-first and includes the absolute full-log path on failure.
• A one-command demo fixture documents both legacy and opt-in invocations.
• The opt-in can be disabled without reverting code.

DAG

This is the demo milestone. Unblocks R7, R8, and R10.

Part of #5858.