fix(embedding): preserve exact decimals in NULL predicates and true joins

[aunjgr]

After #28, MO exact-decimal queries still hit two GPU consumer gaps: IS NOT NULL can send Decimal256's private STRUCT through an unsupported cuDF AST unary operation, and canonical TPC-H Q11/Q15/Q22 retain JOIN ON true as unsupported DuckDB ANY_JOIN because exact preparation skips the ordinary optimizer.

Materialize exact NULL predicates from the canonical top-level validity mask, using the existing task stream/resource and AST temporary-column owner. Lower literal TRUE joins to equal constant keys in the existing GPU join, retaining join kind, projection maps, mark binding, duplicate rows and empty-side outer NULLs. Private key columns remain outside public output. FALSE/NULL predicates retain rejection before readers or GPU work start. The numeric URI, capability mask and ABI-v1 layouts remain unchanged.

Validation at clean head 2a39339beb0c2312dc4b71fbdd472e822450637c, frozen Pixi mo, cuDF/RMM 26.08, RTX 3070:

• Exact and ordinary GPU controls: 14,879 assertions / 11 cases, including all physical widths, NULLs, offset slices, empty input, typed NULL literals and materialize/interpreted/JIT strategies.
• Production C ABI numeric suite: 1,964 assertions / 10 cases. TRUE joins cover INNER/LEFT/RIGHT/FULL, both/either/neither side empty, duplicate/NULL payloads and Decimal256 high limbs; FALSE/NULL controls start no work.
• Binding/preparation controls: 852 assertions / 81 cases. Eight SDK exporter tests and all pinned changed-file hooks pass.
• Clean ABI-v1 SDK: exact source revision, header/literal schema and all 73 artifact fingerprints verified. An independent C99 consumer built from the exported link arguments passes real GPU work, credit accounting, shutdown and reuse.
• Joint MO development consumer: all 22 native preparations and the full public MySQL numeric fixture pass. Its 31 terminal events confirm GPU tasks, capability 31, exact profile, no fallback and zero retained input/result credit. Values, metadata, numeric error ownership, masked errors, prepared division and empty/all-NULL scalar joins are checked against native MO.

The joint MO result uses an explicitly marked development SDK. C still pins merged #28 and must advance only after this PR merges, then repeat merged-pin delivery checks. This is preparation/small-fixture evidence; SF1/SF10, lifecycle/resource baselines and performance acceptance remain PR D. No rollout gate is waived. Local validation used matching official NVIDIA 615.71.09 libraries in a temporary test launcher; host packages and shipped artifacts were unchanged. CI status is reported separately from these local passes.

Refs MO #28968, MO #28966. Contract and ownership map: docs/mo-exact-null-predicates.md.

Consumer integration is available as draft MO C #29757.