feat(sql): expose native JOIN hint adapter seam

[trakhimenok]

Summary

Add an opt-in native JOIN hint contract to dalgo2sql. A trusted adapter can receive the complete validated relation tree, acknowledge every hinted edge, and pass dialect-specific per-JOIN, SELECT, or query fragments to its own complete structured-query compiler. An adapter-owned eligibility hook controls native planning. The SQLite compiler and default SQL bytes remain unchanged.

This seam does not implement SQL Server or Oracle dialect translation; those adapters must provide and test their own compiler and hint policy.

Verification

• Full GOWORK=off go test ./... suite passed with 91.0% coverage.
• DALgo read-transaction test proves eligibility, ordered outer/nested/sibling hints, translation, and compiler handoff.
• Rejection tests prove invalid/unacknowledged paths fail before SQL execution.
• Local multi-module SQLite suite passed using the unmerged SQL branch through a temporary Go workspace.
• WB hooks passed; independent adversarial review approved after strengthening multi-preference order assertions and path-specific errors.