Fix PSB additional layer info length parsing

[SLP-DEV1]

Summary

Fixes PSB Additional Layer Information parsing so tagged-block length width is selected by key instead of treating every PSB tag as 64-bit.

Changes

• use Adobe's documented 8-byte PSB key list
• keep ordinary PSB tagged blocks such as luni, SoLd, and lsct on 4-byte lengths
• preserve existing 64-bit PSB layer/mask, layer-info, and channel-data length handling
• add unit coverage for 4-byte PSB tags, 8-byte PSB tags, and PSD behavior

Closes #20