docs: clarify non-finite float sync behavior (NaN/±Infinity)

[BABTUNA]

Summary\nFollow-up to #6339 (drop json5 dep, handle non-finite floats in socket parse fallback) to make the user-visible behavior explicit in docs/tests.\n\n## Problem\nThe parser fallback behavior for non-finite floats is subtle and easy to regress:\n- Infinity / -Infinity should round-trip to frontend JS numbers.\n- NaN is preserved via sentinel+reviver behavior in the fallback path.\n\nWithout a clear documentation note and explicit compatibility framing in tests, future refactors may accidentally change rendering/round-trip behavior.\n\n## Proposed follow-up\n- Add a short docs note in state/base-vars docs describing expected handling of NaN, Infinity, and -Infinity.\n- Add an explicit compatibility comment around the special_floats integration assertion to codify expected UI representation.\n\n## Acceptance criteria\n- User-facing docs mention non-finite float behavior and compatibility expectations.\n- Integration test intent is explicit for maintainers reviewing future parser changes.\n

[linear]

ENG-9411