Add reverse-dependency CI against the dialect packages

[nbenn]

Summary

• Runs nbenn/sqlr.postgres and nbenn/sqlr.sqlite test suites against the sqlr built from the branch under test, so a core change that breaks a dialect fails here rather than in a downstream repo hours later.
• Core's own suite dispatches on a test dialect and touches no database. That is deliberate, but it means core CI cannot observe a broken dialect at all — this closes the gap without adding a database dependency to the package.
• Two guards exist because the obvious failure mode of a revdep check is passing while testing the wrong thing. One asserts the branch build actually replaced the Remotes: nbenn/sqlr copy pulled from main; the other asserts the Postgres service is reachable, since the round-trip tests skip themselves when it is not and a skip is indistinguishable from a pass in the summary line.
• Modelled on BristolMyersSquibb/blockr.ci, with three deliberate departures: it is a plain workflow rather than a reusable one (a single upstream, not an org of them), it triggers on pull_request rather than merge_group (this repo has no merge queue, so merge_group would never fire), and it runs the dialect suites rather than a full R CMD check on each.

The revdep-all aggregator gives branch protection one stable check name, so adding a dialect to the matrix does not mean editing the required-checks list.