[tomiiiiiiiiiiiiii]

This PR contains a small, conservative set of build-compatibility fixes for current toolchains.

What it changes:

• adds shasum -a 256 support to install_db4.sh
• replaces removed Boost.Filesystem is_complete() calls with is_absolute()
• adds the missing include
• replaces deprecated configparser.readfp() with read()
• namespaces Garlicoin's local scrypt endian helper functions to avoid SDK symbol collisions

The scrypt changes only rename local helper functions. The algorithm and data flow are unchanged.

Intentionally unchanged:

• consensus rules
• Allium / PoW
• difficulty and block rewards
• block/transaction serialization
• wallet.dat / database format
• P2P protocol behavior
• wallet coin selection behavior

This is deliberately a small subset of the broader compatibility work discussed in #96.

[tomiiiiiiiiiiiiii]

Superseded by #99.