Native window aggregation: support Flink split-distinct physical plans

Follow-up to the completed base window COUNT(DISTINCT) support in #97. On main aea6056352c402e0c7008f6f223c7574324d796a, enable table.optimizer.distinct-agg.split.enabled=true and execute this shape over a runtime source with event-time watermarks:

SELECT k, window_start, window_end, COUNT(DISTINCT s), SUM(v), COUNT(*)
FROM TABLE(TUMBLE(TABLE src, DESCRIPTOR(rt), INTERVAL '10' SECOND))
GROUP BY k, window_start, window_end;

TUMBLE, HOP and CUMULATE comparisons all match released Flink 2.2.1 through full fallback. Even with TWO_PHASE requested, every generated plan records:

Calc: unsupported function/operator: HASH_CODE
window aggregate: attached-window aggregation requires two-phase execution
global window aggregate: requires SUM/MIN/MAX/COUNT or paired AVG partials

The corresponding splitDistinct=false paths run native. The unchanged upstream WindowDistinctAggregateITCase contracts currently require fallback for all three splitDistinct=true methods.

Acceptance:

• Support the actual optimized split-distinct stages and their attached/global accumulator layouts, in addition to the separately tracked HASH_CODE scalar.
• Preserve NULLs, duplicates across buckets/partials, mixed ordinary aggregates, temporal bounds, watermarks and late rows.
• Verify state cleanup and memory/RocksDB recovery for each admitted layout.
• Require native work in every necessary stage of the three unchanged upstream methods and their backend variants.
• Retain precise fallback for any unverified form and update the window coverage page.

Retracting DISTINCT remains tracked by #99; this issue does not duplicate that changelog extension.

HASH_CODE scalar tracking: #165.

Start from the unchanged upstream WindowDistinctAggregateITCase methods and the splitDistinct=true plans described in the issue, using commit aea6056352c402e0c7008f6f223c7574324d796a. Trace the attached-window and global aggregate layouts, then verify native execution across the listed backends and cases, with precise fallback for unverified forms and the window coverage page updated.