The intent hold falls back to the arm's real settle time (1 s, not 2 s) when the motion gives no duration; arrival stays at 12 ticks or less #518

From #439. The servos now settle in 150–330 ms (the steward's table), but Intent in src/ui/intent.js still keeps an unreached target for a fixed 2000 ms whenever release gets no motion-derived hold.

Done when:

• [ ] The default hold in src/ui/intent.js is 1000 ms, a comment names where the number comes from, and near stays at 12 ticks or less.
• [ ] A release that passes a motion-derived hold still uses that hold. Nothing else in the page changes.
• [ ] src/ui/intent.test.ts has a test that fails on 2000: an unreached target with no given hold is lost after 1 s.
• [ ] bun --cwd Software/so101 test passes.

Files: src/ui/intent.js, src/ui/intent.test.ts. Risk: more snap-backs if a joint settles slower than 1 s. If #509 (jams) touches the same file, rebase onto it. Brief: any builder, small.

Parked by the crew: the preview's deploy failed and the decider saw no point in another try (pull #535 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.