Build early_switch_fraction into the sci-fi relay + one-click demo page

[jacklange22]

Summary

Makes the two-segment sci-fi waypoint relay demo actually work and look cool, and strips its GUI to one-click.

Three root problems this fixes:

1. early_switch_fraction did nothing. The relay emitted a sparse trajectory (one bus write per waypoint) and relied on a lucky profile_velocity to leave the servo mid-motion. The "almost reaching / changing its mind" feel was non-deterministic — "it isn't working well."
2. Live runs failed precheck. The 0.35 cm preset needs ~221 ticks of travel (top-routing compensation nearly doubles it) but the soft cap defaulted to 200, so the trajectory was rejected before the bus was ever written.
3. The page exposed ~20 parameters (pattern, amplitude, cycles, ramps, coupling, tick caps, currents, profile) that an operator should never touch for a demo.

What changed

• sci_fi_waypoint_relay.build_relay_trajectory now streams a dense overlapping-smoothstep blend through the waypoints. early_switch_fraction sets how much consecutive transitions overlap: 1.0 settles on every waypoint; lower rounds the corners so the spine never fully settles (the sci-fi flow). Deterministic and independent of profile_velocity. Total span stays = video_duration_s; every sample clamped to ±amplitude. Verified avg distance-to-waypoint: 0.0 (esf 1.0) → 0.063 (0.72) → 0.184 cm (0.5).
• two_segment_slow_motion_demo.setup() auto-sizes the soft/hard/step tick caps from the amplitude (tick_budget_for_segment_amplitude_cm) for the relay, so a live run clears precheck first try (0.35 cm → cap 548, actual max 221). Caps are only ever raised. command_rate_hz is now the dense sample + bus-write rate (default 15 Hz).
• GUI page reduced to: two one-click presets ("20s Sci-Fi Spine (curated)" + "Random Weird Spine"), a dry-run / return-to-neutral toggle, and a live preview. The Run button now states the mode — "Preview (DRY RUN — robot will NOT move)" vs "RUN LIVE — move the robot" — so a dry run is never mistaken for a live one (the Build early_switch_fraction into the sci-fi relay + one-click demo page #1 "nothing is running" confusion).
• example.yaml defaults to the curated relay so the page loads a known-good demo with nothing to tune.

Why

It's a thesis-presentation demo: it needs to run first try and look cool. The early-switch knob is now the actual visual control, the caps can't block a live run, and the operator just clicks a preset → preview → un-check dry-run → run.

Test plan

• tests/test_sci_fi_waypoint_relay.py, tests/test_two_segment_slow_motion_demo.py, tests/test_two_segment_motion_patterns.py, tests/test_experiment_framework.py — 193 passed
• Updated tests for the dense design; added one asserting early_switch_fraction actually shapes the path
• End-to-end dry run: 303-point trajectory, all artifacts incl. sci_fi_waypoint_preview.png render from the dense trace
• Live-precheck math verified: max compensated delta 221 ≤ auto-cap 548 ≤ hard cap 600
• Hardware: live run on the rig (dual_segment + accepted all-8 startup) to confirm the flow looks right on camera

Reviewer notes

• Scoped to the slow-motion/sci-fi-relay demo only. An unrelated in-progress change to two_segment_workspace_repeatability_outputs.py was deliberately left out of this branch.

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