Fix Windows tunnel runtime support

[d42me]

Summary

• add Windows frpc downloads with verified checksums and zip extraction
• select frpc.exe on Windows while preserving existing Unix binary behavior
• replace POSIX-only fcntl startup polling with the existing pipe-drain thread path so tunnel startup can detect frpc success/failure on Windows

Stacked on

• Fix Windows CLI startup when tunnel imports are unavailable #701

Tests

• uv run pytest packages/prime-tunnel/tests packages/prime/tests/test_windows_cli.py -q
• uv run ruff check packages/prime-tunnel/src/prime_tunnel/binary.py packages/prime-tunnel/src/prime_tunnel/tunnel.py packages/prime-tunnel/tests/test_binary.py packages/prime-tunnel/tests/test_tunnel.py
• uv run ruff format --check packages/prime-tunnel/src/prime_tunnel/binary.py packages/prime-tunnel/src/prime_tunnel/tunnel.py packages/prime-tunnel/tests/test_binary.py packages/prime-tunnel/tests/test_tunnel.py
• uv run ty check packages/prime-tunnel/src/prime_tunnel/binary.py packages/prime-tunnel/src/prime_tunnel/tunnel.py packages/prime-tunnel/tests/test_binary.py packages/prime-tunnel/tests/test_tunnel.py
• PRIME_DISABLE_VERSION_CHECK=1 uv run prime --help
• PRIME_DISABLE_VERSION_CHECK=1 uv run prime tunnel --help

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Note

Medium Risk
Changes tunnel bootstrap and binary install paths on all platforms (startup ordering and log capture), though behavior is intended to stay equivalent on Unix; new external downloads add supply-chain surface on Windows.

Overview
Adds Windows support for the frpc client used by prime-tunnel.

Binary delivery: Windows amd64/arm64 release zips are downloaded with SHA256 verification, extracted via zipfile, and installed as frpc.exe. Platform detection now normalizes AMD64/ARM64 on Windows; Unix tar.gz paths still resolve to frpc.

Tunnel startup: Connection readiness no longer relies on POSIX fcntl non-blocking reads. Startup begins pipe-drain threads earlier, captures stdout/stderr into _output_lines while _capture_startup_output is set, and _wait_for_connection scans that buffered output for success/failure strings—so the same logic works on Windows.

Tests cover Windows arch mapping, frpc.exe path selection, zip extraction, and success detection via the drain path.

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