Package an existing UI Bundle (2GP)

How to take a UI Bundle that already exists in the current project (under <packageDir>/uiBundles/<name>/, where <packageDir> is the package directory from sfdx-project.json — commonly force-app/main/default) and ship it as a second-generation package (2GP), then install / upgrade / uninstall it in another org.

This is reference knowledge, not a runbook to execute top-to-bottom. The user already has a project and a built (or buildable) bundle. Read their intent and apply only the matching part:

Answer only what was asked. Give the commands for the one part the user needs plus the org each targets and any genuine caveat — nothing else. Do not restate the other parts, re-explain the flavor table, or replay the full build→create→install→promote sequence when the user asked about a single step. A debug question wants the fix, not a packaging tutorial; an install question wants the sf package install line and the subscriber-vs-Dev-Hub distinction, not Part 1 and Part 2. Brevity is correctness here.