Add queued execution speed and explicit pause/resume

[Jepson2k]

Make queued execution speed a controller capability. set_execution_speed(0.1..1.0) retimes the existing planned joint path; pause() decelerates to a hold with the queue retained, and resume() uses the selected positive speed. Changing speed while paused keeps the hold. Fresh readback reports selected, requested, and applied speed separately.

The player interpolates the original joint path before step quantization. Rate changes are checked against acceleration derived from the emitted positions, with preallocated buffers in the control loop. Queued dwell time pauses; positive overrides leave dwell and independent actuator timing unchanged. Jog and servo-stream timing remains separate. Override transitions have their own rate and acceleration bounds; nominal motion-profile jerk ceilings are not guaranteed during a transition.

Standalone completion waits retain wall-clock deadlines. Unconfirmed blocking motion completion raises TimeoutError; the queued motion remains until completed or stopped. Preview retimes trajectory durations and reports paused operations as unresolved.

Depends on #44 and waldoctl #32. Coordinated branches use feat/execution-speed-override.

Validation: 25 motion-pipeline, hostile-wire-input, and simulated client tests passed; preview checks and lint/type/format checks passed. The acceleration regression reproduced a 20% excess under a fast transition before the emitted-position check fixed it. Validation used simulated execution; physical transition tuning remains unverified.

Companion PRs: waldoctl #32, PAR6 #75, Commander #58.