Add an opt-in V2 scan builder and execution path - #10302

Summary

Introduce a separate V2 scan builder and an opt-in branch in DuckDB and DataFusion so the new scan implementation can land incrementally. With VORTEX_SCAN_V2=1, both engines use vortex_layout::scan::v2::ScanBuilder; otherwise they keep their current path. The flag is read once per process.

This PR targets develop directly and can merge independently of the protocol stack (#10047, #10048, #10049). It establishes the structure for follow-on planner, execution graph, and IO changes. It does not yet execute scans through the new planning protocol.

Changes

• Copy the current builder, lazy stream, and repeated-scan implementation into scan::v2, retaining the existing split tasks for initial behavior parity.
• Add ScanBuilder::from_default and a crate-private configuration transfer that moves every builder option, including selection, natural splits, ordering, concurrency, mapping, limits, and row offsets.
• Select the V2 builder at the DuckDB preparation and DataFusion stream entry points.
• Include parity tests for filtering, row ranges, limits, and repeated execution, plus checks that conversion preserves selection, mapping, concurrency, ordering, and supplied split boundaries.

Validation: the initial CI run passed Rust builds and tests, SQL logic tests, and Clippy; the formatting differences in the new tests are fixed with nightly-2026-09-10. DCO now passes. The first S3 run failed during RustFS bucket setup; service startup passed on retry. A later run exposed the same stale OpenDAL dependency edges in develop's lockfile, so this branch includes current develop and Cargo's three-entry lockfile repair. cargo metadata --locked --format-version 1, scoped formatting, and git diff --check pass locally. CI is rerunning on the updated branch.