Fix #906: execute viral propagation only at combination points (main, DuckDB-only)

[javihern98]

Summary

DuckDB-only adaptation for main of the 1.9.X change in #907 (issue #906). Viral attribute propagation was executed (and required) on essentially every dataset operator; per the VTL 2.2 Attribute propagation rule it must run only when input data points are combined, and otherwise copy viral attributes unchanged.

• Combination points (rule executed, rule required): an operation over two or more datasets (dataset–dataset binary, joins, if/case with ≥2 dataset branches); an aggregation/analytic group-by/partition; the hierarchy roll-up.
• Everywhere else (viral copied unchanged, no rule required): unary ops, dataset⊕scalar, calc/keep/drop/rename/unpivot, cast, time shifts, nvl, set operators, check_datapoint/check_hierarchy.

The SemanticError 1-3-3-6 moves from a global per-statement check into the combining operators' validate methods (still fires during semantic_analysis()), flagging only the viral attributes actually combined. The row-preserving DuckDB SQL emissions become plain copies (vp_dataset_wide_sql removed); nvl copies its primary operand (COALESCE-filled).

main is DuckDB-only, so there is no pandas evaluate/apply_row_preserving path (this is the difference from the 1.9.X PR #907).

Fixes #906.

Checklist

• Code quality checks pass (ruff format, ruff check, mypy)
• Tests pass (pytest) — full suite: 5056 passed, 11 skipped
• Documentation updated (error 1-3-3-6 description; error_messages.rst regenerates from messages.py)

Impact / Risk

• Behavior change. Row-preserving operators no longer execute the rule: an aggregate rule no longer collapses a viral column dataset-wide (e.g. abs(DS_1)), and an enumerated rule no longer remaps values — both now copy. A viral attribute never combined no longer requires a rule.
• Supersedes Window check_datapoint viral aggregate over the whole validation result #897. Window check_datapoint viral aggregate over the whole validation result #897 made check_datapoint aggregate the viral attribute over the whole validation result; as a row-preserving validation it now copies the viral value per datapoint. The 13-1 expected output is updated accordingly. Please confirm this is the intended direction.
• No public API changes. No SDMX compatibility concerns.

Notes

• Mirror of Fix #906: execute viral propagation only at combination points #907 (1.9.X), adapted to main's DuckDB-only architecture. nvl is single-operand for propagation.
• The unrelated main.py working-tree change is excluded.