Fix UnityFS block info flags when saving bundles

[snowyegret23]

Problem

Saving UnityFS bundles with UnityPy could produce corrupted block info metadata. In some cases (e.g., Unity 2019.3.x),
UnityPy wrote header-only flags into block info entries. This could cause block info size/parsing mismatches on reload
and lead to struct.error during BundleFile.read_fs().

Root cause:

save() passed block_info_flag=64 for the “none” packer, and save_fs() used that flag directly for block info entries.
The 0x40 bit is a header/layout flag (BlocksAndDirectoryInfoCombined) and should not appear in block info compression
flags. Block info entries should only carry compression-type bits (0x3F).

Changes:

• Default “none” packer now uses data_flag=0x40 and block_info_flag=0.
• save_fs() masks block_info_flag with 0x3F before building block info.
• Each block info entry writes block_flag & 0x3F.

Impact:

• Prevents invalid block info flags from being written.
• Fixes reload failures caused by block info size mismatches.
• Behavior is unchanged for original, lz4, and lzma packers except for safe masking.

Testing:

• Reproduced load failure on Unity 2019.3.x AssetBundle when saved with packer="none".
• After the fix, saved bundles reload successfully with UnityPy.