F-20: Orchestrator: taking over a moving loco starts from speed 0 #23

Severity: HIGH · Effort: Medium · Batch: 1 · Status: Code done in #21 (7c980f9), awaiting bench verification

Found in the second full review of the firmware. Recorded with the rest of the pass (F-19…F-44) in docs/REVIEW_REMEDIATION_PLAN_2.md, which is where its status is kept.

Description

On acquire, Throttle::assignLocomotive zeroes the model and
OrchestratorBackend::acquireLocomotive zeroes its shadow. The loco's actual state arrived in
the opening STATE_SNAPSHOT (and in every LOCO_STATE since), but onLocoState only routes
addresses that are already assigned, so it was discarded. Taking over a loco another operator
or an automation run has at speed 60, one click clockwise commands speed 4; one click
anticlockwise commands reverse 4.

WiThrottle does not have this problem: JMRI restates speed and direction after M…+.

Plan

1. OrchestratorClient keeps a compact cache of the latest state per address, filled from
   STATE_SNAPSHOT (which replaces it wholesale) and LOCO_STATE, cleared when the link goes
   down. Bounded in size.
2. OrchestratorBackend::acquireLocomotive seeds its shadow from the cache and replays the
   cached state through the throttle-state callback, exactly as a LOCO_STATE would arrive.
   This is display-side use of layout state: nothing is sent outward, so the "a snapshot is
   never replayed as commands" rule stands.
3. Unit test: snapshot, then acquire, then assert the update carried the snapshot's speed and
   direction.

Files: OrchestratorClient.h/.cpp, OrchestratorBackend.cpp, tests

Acceptance Criteria

• Acquiring a loco the orchestrator already reports as moving shows its speed and
  direction immediately (replayed through the throttle-state callback).
• The next knob click moves from that speed, not from 0 — the controller's model and
  the backend's shadow are both seeded.
• Nothing is sent to the orchestrator on acquire.
• Unit tests test_orch_backend_acquire_seeds_from_layout_state,
  test_orch_backend_acquire_of_unreported_loco_shows_nothing and
  test_orch_snapshot_replaces_cached_state pass on the board (they compile).
• Bench: drive a loco from the web UI, take it over on the device, click once each way.