feat: parallelize FDSN provider station queries per box

[jsteinberg34]

Summary

• Replaces the sequential for provider in providers loop in download_boxes() with a ThreadPoolExecutor so all FDSN provider station queries for a given box fire concurrently
• Extracts _query_provider as a module-level worker function
• Bumps version to 0.2.1 and checks off the roadmap item in README

Background

For each download box, the old code queried each FDSN provider (EARTHSCOPE, SCEDC, etc.) sequentially with blocking client.get_stations() HTTP calls. With N providers, the station-query phase cost N × latency per box — pure network idle time, since providers are completely independent servers with no data dependency between them.

The new approach fires all queries concurrently, reducing per-box station-query time from N × latency to ~latency. MassDownloader (which dominates total runtime) is untouched — it already parallelises internally and runs after all queries complete.

Implementation notes

• max_workers=max(1, len(clients)) guards against the edge case where all providers fail to initialise and clients is empty — ThreadPoolExecutor(max_workers=0) raises ValueError
• Each provider uses its own Client instance initialised before the box loop, so there is no shared mutable state between threads
• Each worker writes to a unique path ({provider_name}_stations.xml), so no file contention
• Error handling is preserved: FDSNNoDataException → skip silently, any other exception → log and skip

Test plan

• 126/126 existing tests pass with no changes
• test_download_boxes_tolerates_get_stations_errors[nodata] — FDSNNoDataException path
• test_download_boxes_tolerates_get_stations_errors[generic] — generic exception path
• test_download_boxes_tolerates_client_init_failure — empty clients dict edge case

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/groundtrack/download.py	71.42%	3 Missing and 3 partials ⚠️

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