Make showcase launch independent of shell setup

[thunderballfists]

Setup instructions relied on a globally available textui command, so the showcase failed when the project was installed in a different environment. Add an executable ./showcase checkout launcher that installs the locked project with isolated Poetry 2.4.3 on Python 3.12 and runs python -m textui.

The launcher resolves its own checkout path, ignores activated environments and stale checkout .venv directories, stops on installation failure, and preserves the application's exit status. README and example instructions use the same environment policy; the source distribution includes the launcher and lockfile. This delivers roadmap unit 1A.

Validation:

• 436 headless tests passed, including five launcher subprocess regressions.
• Poetry lock validation and wheel/source builds passed.
• Pyflakes, shell syntax and Git whitespace checks passed; all six local visual checks passed.
• Source archive contains the executable launcher, lockfile and showcase; the library wheel remains unchanged in scope.
• Real terminal launch from a fresh clone, called by absolute path from /tmp, passed and Ctrl+Q exited with status 0. Confirmed Python 3.12 and the fresh clone's installed module were used while a stale checkout .venv and unrelated active-environment settings were ignored. Repeated after deliberately saving a Python 3.14 selection; the launcher switched back to 3.12 and Ctrl+Q again exited cleanly.

Independent review found and verified the saved-interpreter fix: explicit Python 3.12 selection precedes installation. GitHub review feedback about stale .venv/bin/python visual commands was fixed, verified and resolved. Final CI and feedback are checked before merge.

[chatgpt-codex-connector]

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