#21620 fix(cmux-next): stabilize new-tab motion (current base)

Focused new-tab motion fix on current feat-cmux-next base.

Validation: focused new-tab tests 10 passed, 31 assertions; diff-check clean.

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Summary by cubic

Stabilizes the new-tab list motion so it no longer remeasures row geometry or replays animations when the owner rerenders with the same row keys. The motion hook now keys off a string signature, a stable list root, and the row selector, and it re-observes a replaced root.

• Removed the explicit data-agent-motion="off" check; reduced-motion behavior now depends only on CSS variable overrides.
• Rerenders with identical row sets now skip all geometry reads for both normal and reduced-motion paths.