feat: add cursorSpeed to control how fast the cursor travels

Summary

Cursor travel is timed from the distance covered (moveDuration in cursor-motion.ts, and computeDragTiming for drags), tuned to feel like a human hand. There is no way to adjust it. On a dense config the cursor spends a lot of the runtime gliding, and the only lever today is speeding up the finished file in post, which speeds up typing and deliberate pauses along with it.

cursorSpeed scales the travel duration and nothing else:

{
  "cursorSpeed": 1.75,
  "videos": {
    "hero": {
      "url": "/",
      "cursorSpeed": 1,
      "steps": [{ "action": "click", "text": "Get Started" }]
    }
  }
}

2 covers the same ground in half the time, 0.5 takes twice as long. Typing speed (charDelay), click dwell (clickDwell) and step delays (delay / defaultDelay) are unaffected, so the pacing of the demo itself is preserved while dead travel time shrinks. It sits alongside clickDwell: settable at the top level, per video, and inherited the same way.

The multiplier lives on RecordingContext, so both plain moves and drags pick it up, and the eased path and jitter are unchanged: only the duration scales.

Test plan

• pnpm test (106 core, 112 webreel), pnpm type-check, pnpm lint, pnpm format:check
• New tests: context default and clamping of nonsense values, and drag timing halving at cursorSpeed: 2
• Recorded the same 22-step config at three speeds, everything else held constant:

Typing, click dwell and step delays are identical across all three; only travel time changes.

No changeset here, per the release process in AGENTS.md; happy to send one in a follow-up PR.

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