Add motion policy tests and fix settings pane height

[austin-smith]

What Changed

• New MotionPolicyTests pinning the animation speed contract: the catalog's stored scales, from(rawValue:) falling back to Normal on an unknown id, the Reduce Motion clamp overriding every speed, duration(_:) math, isInstant, and the animation builders returning nil (apply without animating) whenever motion is off.
• macOS General settings pane height corrected from 410 to 355 — the 410 shipped in Add animation speed setting #76 left roughly 70 points of dead space under the Gameplay group.

Why

MotionPolicy shipped in #76 without unit tests; everything else its size in the repo is covered, and the raw-value fallback in particular protects old saved defaults against a future option rename. The pane height in #76 was set without visual verification; 355 is measured from a screenshot of the rendered window (content spans 335pt below the toolbar divider plus the form's bottom inset).

Validation

• Full unit suite on macOS: 568 tests including the 9 new ones, 0 failures.
• macOS build relaunched with the corrected height for visual confirmation.

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