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

[BABTUNA]

Summary

Follow-up to #6339 (drop json5 dep, handle non-finite floats in socket parse fallback) to make the user-visible behavior explicit in docs/tests.

Problem

The parser fallback behavior for non-finite floats is subtle and easy to regress:

• Infinity / -Infinity should round-trip to frontend JS numbers.
• NaN is preserved via sentinel + reviver behavior in the fallback path.

Without a clear documentation note and explicit compatibility framing in tests, future refactors may accidentally change rendering/round-trip behavior.

Proposed Follow-up

• Add a short docs note in docs/vars/base_vars.md describing expected handling of NaN, Infinity, and -Infinity.
• Add an explicit compatibility comment around the special_floats integration assertion to codify expected UI representation.

Acceptance Criteria

• User-facing docs mention non-finite float behavior and compatibility expectations.
• Integration test intent is explicit for maintainers reviewing future parser changes.

[linear]

ENG-9411