fix: account for NaN in float stats exported to DuckDB and DataFusion

[jackylee-ch]

What

Vortex's min, max and sum statistics skip NaN, but the DuckDB and DataFusion integrations report
them to the engine as exact bounds over every value. On a float column holding NaN, both engines
then answer from the statistics:

• SELECT max(x) returns the largest non-NaN value instead of NaN;
• WHERE x = 'NaN' returns no rows (DuckDB plans an EMPTY_RESULT, DataFusion prunes the file);
• in DataFusion, where a negative NaN sorts below every value, WHERE x < 0.5 drops it as well.

Fix

Check the footer's exact NaNCount. DuckDB sorts NaN above every value, so its max becomes NaN
when the count is non-zero and is left out when the count is unknown; min is already correct.
DataFusion sorts NaN by sign, so min, max and sum are reported only for a NaN-free column (this
also guards the v2 source, which reports no statistics yet). Other columns are unchanged; float
columns in files from before NaNCount (0.29 to 0.32) lose their bounds.

The new NaN cases fail on develop (max(x) returns 2, the NaN filters return no rows); two plan
checks pin that NaN-free float columns are still answered from statistics.

constant_distinct_count (from #10448) now reuses is_nan_free. #10265 and #9775 rewrite these
functions.

AI assistance

Written with agentic AI assistance; each failure above was reproduced on develop before the fix.