fix(numeric): preserve opaque decimal root column names

[aunjgr]

MO Q1 native preparation read past its SQL root names because Decimal256's private four-limb STRUCT carrier was counted as nested SQL fields. Mark canonical exact-decimal carriers opaque through the query-scoped importer policy, so each remains one result column and following aliases keep their positions.

Pins merged importer #5, revision 99c7ca3b6f8f3159239e119ed2982d42f98c4690. The callback validates the full canonical carrier through the existing descriptor decoder. The regression binds Decimal64/128/256 at precision 15 followed by an independent BIGINT result, asserting both headings and the exact physical descriptor; ordinary STRUCTs remain outside this policy. Numeric arithmetic, GPU kernels, C ABI layouts and capability mask 31 are unchanged.

Validated at head 433cadd43a10442b0825a23234fb6d859207e9d5, frozen Pixi mo:

• Clean incremental build, native binding suite: 852 assertions / 81 cases passed.
• Production exact-numeric C ABI: 1,468 / 8 passed, including aggregate widths/NULLs, scalar-versus-aggregate/cast errors, conditional masks, exact joins, Decimal256 ordering and healthy reuse.
• Independent C consumer: passed real GPU execution, source/result credit and runtime lifecycle checks.
• SDK exporter: 8 tests passed. Regenerated SDK records the exact clean head (source_dirty=false); all 73 artifact fingerprints, ABI/header and canonical literal schema match.
• All repository-pinned changed-file hooks passed. Refreshed actual base upstream-dev-merge; zero commits behind; self-review found no in-scope blocker.

The merged importer tree matches the tested fix. Its canonical importer regressions previously passed 157 / 3, and joint development binding coverage passed 1,009 / 84. Importer distribution CI's seven failures were classified against its exact clean base, including a prior STRUCT field-selection failure/crash and five independently reproduced later failures; no assertion was weakened or skipped. This does not claim full distribution or compiler CI is green; current-head CI is tracked separately.

MO C is saved locally and will pin this PR only after it merges, then rerun all-22 native preparation and full public numeric validation. The parity campaign D follows merged, validated C. Refs matrixorigin/matrixone#28968 and matrixorigin/matrixone#28966.