cli: add hflow curate --dry-run; fix manifest: None in the report

[chintondutta]

Fixes #8.

Two related rough edges

1. The library supports dry runs, the CLI couldn't reach them. curate(...) already documents: "With output=None the query still runs (row count + coverage reporting) but nothing is written." But --output always defaulted to ./data/manifest.parquet with no way to opt out.
2. The report rendered the None case badly. CurationReport.summary() printed the literal manifest: None (12 rows, from 40 cataloged episodes) when no manifest was written.

Change

• Added --dry-run to the curate subparser, in a mutually exclusive group with --output (add_mutually_exclusive_group(), as suggested in the issue). _command_curate passes output=None to curate() when set.
• CurationReport.summary() now branches on manifest_path is None and renders manifest: (not written; dry run) instead of the None literal.

Testing

Two new tests in tests/test_catalog_curation.py:

• test_cli_curate_dry_run_prints_without_writing: hflow curate ... --dry-run prints row count and coverage, and the summary reads manifest: (not written; dry run) with no literal None anywhere in the output.
• test_cli_curate_rejects_dry_run_with_output: --dry-run --output x.parquet together raises SystemExit(2) from argparse (matching the existing pattern for --profile in test_ingest_rejects_unknown_profile).

uv run pytest -q       # 299 passed, 3 skipped; unrelated: tests/test_ffmpeg.py fails/errors in this sandbox (no ffmpeg/ffprobe on PATH)
uv run ruff check --fix
uv run ruff format
uv run ty check

[github-actions]

👋 Hi @chintondutta — thanks for the contribution! To keep starter issues available
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