Share downloaded bitcoind path with build script

[DeepCogNeural]

Fixes #47.

Summary

• move the downloaded bitcoind executable path construction into a small shared helper
• use that helper from both build.rs and the public downloaded_exe_path() API
• make the build-script existing_filename check share the Windows bitcoind.exe suffix behavior

Verification

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo check
• BITCOIND_SKIP_DOWNLOAD=1 cargo check --features 26_0
• BITCOIND_SKIP_DOWNLOAD=1 cargo test downloaded_exe_path_uses_bitcoin_layout --features 26_0
• cargo test --no-run
• BITCOIND_SKIP_DOWNLOAD=1 cargo test --no-run --features 26_0
• git diff --check