Operator speed control + max-rate (50 Hz) relay sampling

[jacklange22]

Summary

Follow-up to #1/#2. Adds the two things requested:

1. Speed control — a Speed slider on the demo page. Drag right for faster, left for slower/cinematic. It maps inversely to the relay's video_duration_s over [6 s fastest .. 45 s slowest] and shows the resulting "~N s per loop". (For this demo, speed = how fast the spine traverses the whole path = the motion duration; the servo profile stays un-throttled so it tracks the path at whatever speed you set.)
2. Max-rate sampling — the dense sample + bus-write rate is raised to 50 Hz everywhere (preset, YAML, config defaults). 50 Hz is the realistic 8-servo sync-write ceiling — the highest rate that reads as smooth without making the demo run in slow motion (above the bus's sustainable write rate the absolute-paced loop falls behind a schedule it can't meet).

Also fixes the now-stale speed advisory: the relay streams one dense write per sample (not waypoint_count), so it now warns only above ~55 Hz and explains the slow-motion failure mode.

Test plan

• tests/test_two_segment_slow_motion_demo.py, tests/test_sci_fi_waypoint_relay.py, tests/test_two_segment_motion_patterns.py — 149 passed
• Speed mapping verified: slider 100 → 6 s, 1 → 45 s, 50 → ~26 s; syncs back from a 20 s config to slider 64; preview shows ~1000 bus writes @ 50 Hz
• Hardware: confirm 50 Hz is smooth and the speed slider feels right; if the bus can't sustain 50 Hz the motion will run slow → drop command_rate_hz

Reviewer notes

• If 50 Hz makes the demo run slower than the slider says, the bus topped out below 50 Hz — lower command_rate_hz in the YAML.
• Scoped to the demo; the unrelated in-progress two_segment_workspace_repeatability_outputs.py change is left out.

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