`CEIL`/`FLOOR` of a windowed `DECIMAL` returns unexpected results

[wanteatfruit]

CEIL(d) and FLOOR(d) on a DECIMAL call sql.DecimalCtx.Ceil(num, num) / Floor(num, num), which mutate num in place. Window aggregates (MAX(d) OVER (PARTITION BY d), also MAX() OVER () / ORDER BY) hand every peer in the partition the same *apd.Decimal pointer. Projecting the window column next to CEIL/FLOOR therefore turns d into the rounded value for every row after the first peer.

ROUND does not; BIGINT/DOUBLE windows do not; a one-row partition does not; SELECT * without CEIL does not. index_builder.go in the same module already copies into a fresh *apd.Decimal before Ceil — the scalar function does not.

CREATE TABLE t (id BIGINT, d DECIMAL(10,2));
INSERT INTO t VALUES (1, 12.34), (2, 12.34);

SELECT id, d, CEIL(d)
FROM (SELECT id, MAX(d) OVER (PARTITION BY d) AS d FROM t) s
ORDER BY id;

However, removing the equivalent identity table would make the result correct:

CREATE TABLE t (id BIGINT, d DECIMAL(10,2));
INSERT INTO t VALUES (1, 12.34), (2, 12.34);
SELECT id, d, CEIL(d) FROM t ORDER BY id;
-- Expected/actual: (1, 12.34, 13), (2, 12.34, 13)

Reproducible on Dolt 8.0.31 / v2.2.3-49-ga995f245c @a995f245c, go-mysql-server v0.20.1-0.20260805191915-e5eafe0da809.