Add LocalSafeSearch source for pre-downloaded SAFE dirs / zips

[scottstanie]

Summary

• Re-adds a path for users who already have full-frame Sentinel-1 SAFE products on disk (e.g. fetched from ASF Vertex as .zip archives) and want sweets to feed them straight into COMPASS. The v0.2 rewrite replaced the old ASFQuery + _unzip.py path with burst2safe-only downloads, leaving no way to consume pre-existing zips.
• New LocalSafeSearch(kind=\"local\") source: accepts .SAFE dirs or .zip archives in a user-supplied out_dir, auto-detects the COMPASS using_zipped flag (.SAFE preferred when both are present; explicit override available), no start/end/track required.
• Workflow + CLI wiring: added to the Source discriminated union; _download() validates non-empty and returns what's on disk; _geocode_slcs threads using_zipped through to COMPASS; _dem_bbox applies the 1-deg buffer so full IW frames fit. CLI --source local skips date flags.

Closes #135.

Test plan

• New unit tests (9) in test_core.py / test_cli.py cover model validation, zip-vs-SAFE auto-detect, using_zipped override, YAML round-trip, DEM buffer, --source local CLI assembly, and that non-local sources still require --start / --end.
• Full local test suite passes (pixi run pytest tests/ --ignore=tests/test_download_search.py — 62 passed).
• pre-commit run -a clean (black, ruff, mypy).
• End-to-end smoke run with a real directory of ASF .zip SAFEs through sweets run still needed — I don't have a zip stack handy locally.

Example usage

# sweets_config.yaml
bbox: [-104.0, 32.0, -103.0, 33.0]
search:
  kind: local
  out_dir: /path/to/my/asf/zips

Or via CLI:

sweets config --source local --out-dir /path/to/my/zips --bbox -104 32 -103 33

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