Run View: handle Stop, failures, and changed searches

[5aumit]

What to build

Finish the non-success Run paths and revision flow. A researcher can stop safely, understand a
failure or Empty Results, and revise a Search Brief without confusing old and new Results.

Blocked by

#7 and #8.

Acceptance criteria

• Stop cooperatively produces Cancelled, preserves completed Stage history and Search Brief,
  and never exposes incomplete Results.
• Failed Runs show a plain-language Recovery View and no incomplete Results.
• Empty Results explains that sources completed successfully and offers safe refinement
  guidance.
• Completed with warnings, Failed, Cancelled, and Empty Results are available as offline
  replay cases.
• Entering edit mode preserves the prior completed Results for reference.
• Submitting a revision clears prior Results before the new Run becomes Queued.
• Deterministic and browser E2E tests cover all terminal outcomes and revision behavior.

[5aumit]

Delivery order

This issue remains the next Run View milestone after #8, but should be delivered in two small slices:

1. Failed and Empty Results: preserve the Search Brief and completed Stage history, show a plain-language explanation, expose no incomplete ranking, and add replay plus browser coverage. This also fixes any public contract that calls an active Run completed.
2. Stop, Cancelled, and revised searches: cooperative Stop behavior, safe cancellation, edit mode, and clearing old Results when a revision begins.

Do not start this issue until #8 is complete. The ready-for-agent label is removed because the blocker is active.