Native IF: admit BOOLEAN result branches #210

Rechecked on 2026-09-20 against canonical main 89da01cdad7342c4d825e36eaf4e8dc73bc7af9f, released Apache Flink 2.2.1, JDK 17, default compatibility settings. Each expression was run against independent stock-Flink and StreamFusion-enabled streaming environments with bounded runtime DataStream input (parallelism 1), so source values cannot be folded away. Results and resolved result types agree through Flink fallback; this is a native support gap, not a demonstrated wrong-result bug.

PR #169 closed #164 for its verified result-type scope. BOOLEAN result branches remain unsupported although released Flink executes them.

Runtime schema and row: a BOOLEAN=true, i INT=3, b BOOLEAN=false.

SELECT IF(i < 5, a, b) FROM src;

Stock Flink returns TRUE with BOOLEAN result type. StreamFusion returns the same result through zero native substitutions and Calc: IF requires a verified scalar result type.

Independent runtime condition probes also confirm TRUE/FALSE/NULL select TRUE/FALSE/FALSE respectively, all through the same fallback.

This comes from a runtime adaptation of ScalarFunctionsTest. This ticket concerns the remaining BOOLEAN overload, not the already-shipped STRING/numeric IF coverage.

Cover TRUE/FALSE/NULL conditions, nullable BOOLEAN branches, nested predicates, unselected failing branches, and an all-filtered batch. Verify released Flink's actual BOOLEAN overload/typing contract before changing the whitelist.

Suggested priority: P2, basic scalar control-flow coverage.

Acceptance: add runtime-source SQL parity tests that assert native Calc execution as well as values and resolved types; include NULLs and boundary/error cases; retain safe fallback for unverified forms and update docs/operators/calc-filter.md with the admitted scope.

Latest-main validation: rebuilt the debug native library from this commit and ran 562 targeted runtime SQL cases covering these support gaps and adjacent controls. This issue group contains 4 successful comparisons that still record explicit fallback. Host-rejected/both-error cases are excluded from that count. This validates released Flink 2.2.1; it does not claim execution of the Flink 1.18 profile. No implementation is included.