fix(macapp): fence stale stop and steer actions to rendered run identity

[dennisonbertram]

Problem

Independent review of #1123 found that approval/plan/input now use explicit expected run IDs, but ChatView Stop (run.cancel()), composer steer (run.steer()), and ToolWalk timeout (run.cancel()) still resolve currentRunID dynamically. If A was rendered and B becomes selected before action execution, these can POST to B.

Acceptance criteria

• Add explicit expected-run variants for cancel and steer, with guards before Task/network; UI and ToolWalk capture the rendered/decision run identity.
• Add deterministic stale Stop, stale steer, and ToolWalk-timeout tests proving zero B endpoint request after A -> B transition.
• Preserve legitimate current-run cancel/steer, fix(macapp): acknowledge run-control failures and preserve pending user input #994 single-flight/delayed ACK behavior, external exact-cancel isolation, and fix(macapp): clear stale interactive affordances when external run ownership changes #1122 approval/plan/input fences.
• Sol diagnosis, cheap re-review, strict format/full Swift/repo gate/hosted checks.

Scope

Native run action ownership only; stacked after #1123; no callback behavior. Blocks #1123/#1118 promotion.