ACT/buffer-refresh: no shared #container lock — refresh engine and step machine bypass action-lock

[Zillatron27]

Audit finding (v1.2.0 consistency pass). Two gaps in the single-driver guarantee for `#container`:

• `lib/buffer-refresh/engine.ts` (`executeBufferRefresh`) drives buffers with no coordination with `lib/act/action-lock.ts` — `isActionInFlight` has zero production callers. A manual base refresh started during a driven action (or vice versa) can interleave two DOM drivers.
• `lib/act/runner/step-machine.ts` / `ActionRunner.execute` never acquires the action lock, contradicting action-lock's own contract. Currently latent (ActionRunner has no production callers until Mobile buffer actions: adopt jackinabox86's act engine + buffer navigator #25 Phase D), but Phase D inherits the gap.

Fix direction: route `executeBufferRefresh` through `acquireActionLock`/`isActionInFlight`, and acquire/release in `ActionRunner.execute` (release in `StepMachine.stop`). Touches device-verified working paths — needs its own change + device check, not a ride-along. Should land before #93 (mark-as-read) adds another buffer-driving feature.