* edit LucasArts GOB v2 through its trailing directory

[Hawkynt]

What changed

• add a changed-byte editor for canonical GOB v2 archives whose directory is the exact physical trailer
• batch add/replace writes changed stored payloads where the old directory began, regenerates the directory after them, and patches only the 32-bit directory-offset field in the 12-byte header
• remove rewrites the directory in place and leaves survivor payloads at their original offsets
• wipe replaced/removed payload ranges only when no surviving directory entry overlaps them, so aliased/shared data is preserved
• unsupported non-trailing layouts reject before the first write and keep the verified extract/re-create fallback
• correct the layout map to include the fixed header and trailing directory as metadata instead of exposing only payload extents

Complexity

For canonical trailing-directory GOB v2 archives:

• pure add: O(directory metadata + added bytes)
• replace: O(directory metadata + replacement bytes + safely wiped superseded bytes)
• remove: O(directory metadata + safely wiped removed bytes)

Untouched payloads are neither read nor moved. Holes left by replacement/removal are deliberately reclaimed by defragmentation, not by ordinary mutation.

Tests

Descriptor-level I/O-budget tests use a 4 MiB untouched stored member and require tiny add/replace/remove operations to stay below 128 KiB of archive reads and writes while preserving that member's exact physical offset and bytes. Replacement/removal verify stale-byte wiping, a missing-name removal must perform zero writes, an aliased-payload case proves one directory record cannot wipe data still referenced by another, and the layout test pins header/payload/directory classification.