Add colsUsed to vtable BestIndex callback

[theimpostor]

Split out of #1336 per review feedback.

Summary

• pass sqlite3_index_info.colUsed through to the Go VTab.BestIndex callback
• update the examples and tests for the new callback signature
• add test coverage for the propagated colsUsed mask

Fixes #1323.

[mattn]

Thanks for splitting this out. The feature itself is welcome — colUsed is a useful planner hint and #1323 is a real gap.

My concern is that changing the VTab.BestIndex signature is a breaking API change. Every external implementer of VTab will fail to compile after upgrading, which we want to avoid in v1.

This repo already uses optional interfaces for extensions (see VTabUpdater, EponymousOnlyModule). Could you take the same approach here? Something like:

type VTabBestIndexV2 interface {
    BestIndexV2([]InfoConstraint, []InfoOrderBy, uint64) (*IndexResult, error)
}

In goVBestIndex, type-assert and call BestIndexV2 if implemented, otherwise fall back to the existing BestIndex. That way existing users keep compiling and only those who need colUsed opt in.

Minor: SQLite names the field colUsed (singular) — matching that naming would make the docs easier to follow.