ci: verify sample regeneration and document manifest cleanup

[omer-cengel]

Summary

Release verification now proves that the external sample regenerates repeatably and cleanly: generation works with an explicit configuration path from another working directory, identical input reproduces identical sources and manifest, and a renamed or removed query group leaves no stale repository behind. The quickstart's regeneration guidance now describes the output manifest and its cleanup.

Changes

• scripts/verify-release.sh generates the copied sample with generate --config ../sample/sqlcj.yaml from an empty sibling directory, checks that the repository and sqlcj-manifest.txt are written and that the working directory stays empty, then generates again and requires a byte-identical output tree.
• Two scratch configurations generate the query groups Author and Library and then only Writer; the script requires the old repositories to be gone, WriterRepository.java to be the only generated source, and the manifest to list it alone.
• The existing default-path generation must restore the first generated output byte for byte before the sample is compiled and run.
• The quickstart lists the manifest among the generated files, attributes a stale repository to a renamed or removed sql entry, explains that regeneration deletes it, and keeps mvn clean because compiled classes are never deleted.

Scope and non-goals

• No compiler, CLI, runtime, sample source, or workflow changes; the added checks exercise existing behavior.
• Compiled-class cleanup, incremental compilation, and a generate --check mode are not added; mvn clean remains part of the documented sequence.