feat: add --jdk-distro flag for alternate JDK distributions (Corretto, etc.)

[rustyrazorblade]

Split out of #773 (and supersedes the JDK-distro half of PR #773 / branch feature/jdk-distro-app-arch).

What

Let a user select the JDK distribution (e.g. Amazon Corretto) independently of everything else, via a --jdk-distro flag, so clusters can build/run on a non-default JDK vendor.

Why split from #773

PR #773 bundled two unrelated concerns — JDK distribution selection and mixed-architecture support. Per owner direction (2026-07): arch and distro are separate axes, and the distro axis is being deferred. #773 is being refocused on the --db.*/--app.* namespacing + auto-derived architecture (arch inferred from instance type via the EC2 describe-instance-types API — no explicit arch flag). This issue captures the JDK-distro work that was pulled out so it isn't lost.

Scope (from the JDK-distro half of PR #773)

The relevant machinery already prototyped on feature/jdk-distro-app-arch:

• packer/base/install/install_jdks.sh — install the selected distro
• packer/cassandra/bin/set-java-version, packer/cassandra/bin/use-cassandra
• packer/cassandra/cassandra_versions.yaml, packer/cassandra/install/install_cassandra.sh
• commands/cassandra/UseCassandra.kt, configuration/CassandraVersion.kt
• Related tests

Notes

• Not ready to design/implement yet — filed to preserve the work and keep feat(cli): namespace db/app options (--db.* / --app.*) + auto-derive arch from instance type #773 focused.
• Related: Support amazon corretto jdk distributions #169 (support Amazon Corretto JDK distributions) — likely a duplicate/companion; reconcile when this is groomed.