Native DECIMAL: support FLOOR and CEIL in verified scalar consumers #226

Verified on PR #220 after rebasing onto merged #222: commit 16ebb1e5a45cc5420abf90473b785d6c0b908a82, released Apache Flink 2.2.1, JDK 17, default compatibility settings. Each probe uses independent stock-Flink and native-enabled environments with bounded runtime DataStream input. Successful comparisons check output values and resolved result types before classifying execution.

Suggested priority: P2. This is a verified native support gap; the successful probes return equal results through explicit Flink fallback.

The exact-numeric work in #208 explicitly excluded DECIMAL FLOOR/CEIL. They still block otherwise executable scalar expressions. Casting their result to STRING provides a successful released-Flink baseline.

Runtime probe

Input type:

ROW<`f0` DECIMAL(20, 3), `f1` INT> NOT NULL

Input: [+I[123.456, 2]]

SELECT CAST(FLOOR(f0) AS STRING) FROM src;

Flink result: [[123]]; resolved type: [STRING]. Native substitutions: 0.

Calc: FLOOR: only float/double operands admitted

Runtime probe

Input type:

ROW<`f0` DECIMAL(20, 3), `f1` INT> NOT NULL

Input: [+I[123.456, 2]]

SELECT CAST(CEIL(f0) AS STRING) FROM src;

Flink result: [[124]]; resolved type: [STRING]. Native substitutions: 0.

Calc: CEIL: only float/double operands admitted

Direct FLOOR/CEIL DECIMAL collection with this fixture throws an AbstractBinaryWriter.writeDecimal assertion on both sides; it is not a native-only failure. Verify the inferred precision and writer contract before widening direct-result admission. Cover positive/negative values, exact integers, NULL and large precisions.

Acceptance

Preserve released Flink values, resolved types, NULL behavior and relevant error/short-circuit semantics. Add runtime-source SQL regressions that assert the specific native operator as well as result parity; do not use constant folding as proof of native support. Keep safe fallback for unverified forms and update the corresponding operator coverage documentation.