#358 fix: make SEA binary cache writes concurrency-safe

Summary

• write downloaded SEA binaries to unique temporary files in the cache directory
• atomically publish completed downloads and tolerate another process winning the cache race
• clean up temporary artifacts on both success and failure
• add regression coverage for concurrent writers and existing-cache behavior

Testing

• uv run --frozen pytest tests/test_sea_binary.py -q (6 passed)
• uv run --frozen ruff check .
• uv run --frozen pyright -p .
• uv run --frozen mypy --platform linux .
• full Python 3.9 and 3.14 suites run on Windows; only the pre-existing Windows failures addressed by #355 remained

Fixes #351

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Summary by cubic

Make SEA binary cache writes concurrency-safe. Previously concurrent writers shared a temp path and could race, leading to partial or orphaned files; now each writer uses a unique temp file, publishes atomically, and cleans up. Fixes #351.

• Write downloaded SEA binaries to unique temp files in the cache directory via tempfile.NamedTemporaryFile.
• Atomically replace the destination; if another process already populated the cache, keep that file.
• Always remove temp files, on both success and failure.
• Add regression tests for existing-cache reuse and concurrent writers.