fix(cli): stop before any model call when a locale's persona dataset is missing

[3mei]

What changed and why

Persona datasets are a one-time download per locale. When one was missing, simulate and panel failed inside Data Designer with a file-pattern traceback, and a live simulate got there only after its model health checks. They now stop before any model call and say what to do.

• simulate, panel and simulate --materialize-inputs check each locale's dataset in the folder Data Designer's sampler reads, including when DATA_DESIGNER_MANAGED_ASSETS_PATH or DATA_DESIGNER_HOME moves it. A missing dataset raises a ConfigError (exit 2) that names the expected file and gives one data-designer download personas --locale … command for all of them, with the NGC steps. India language variants map to the en_IN dataset, and a locale Data Designer has no dataset for gets the list of locales it provides.
• simulate --dry-run and panel --dry-run add a personas: line, ready or the same report, and still exit 0, since the offline check comes before the download.
• usersim smoke adds the download step to its next-step hint when the en_US dataset is missing.
• simulate --inputs replays stored personas and skips the check.
• architecture/cli.md describes the check.

How it was verified

• make check-all: lint clean, 333 files already formatted.
• make test: 4083 passed. New: tests/test_cli_persona_datasets.py (12 tests) and one test in tests/test_smoke.py, on the suite's en_US-only persona fixture. Each fails without the change.
• make smoke, make check-license-headers, make check-doc-links, make check-notebooks-clean and make check-dist pass.
• From an empty home directory, usersim simulate --locale en_US --locale ta_Taml_IN --locale en_GB --num-rows 1 --out o exits 2 before any model call. It names en_US and en_IN with data-designer download personas --locale en_US --locale en_IN, and lists the locales Data Designer provides for en_GB. The same command with --dry-run reports this and exits 0.