* edit UnityFS BlocksInfo-at-end bundles by appending tail blocks

[Hawkynt]

What changed

• add a changed-byte editor for modern UnityFS bundles with BlocksInfoAtEnd (0x80)
• pure additions append new independent Stored storage blocks where the old trailing BlocksInfo record began, then regenerate only BlocksInfo and the four mutable size/flag fields in the header
• existing compressed storage blocks stay opaque and byte-identical; they are neither decompressed nor moved
• direct removal is allowed when removed non-empty nodes occupy a suffix of whole storage blocks; zero-length nodes are metadata-only removals anywhere
• same-name replacement, front-loaded BlocksInfo, or removals that share a storage block with surviving nodes reject before the first write and retain the verified rebuild fallback
• advertise CanModify now that a genuine random-access mutation path exists for the supported profile

Complexity

For BlocksInfoAtEnd layouts:

• pure add: O(BlocksInfo metadata + added bytes)
• zero-length remove: O(BlocksInfo metadata)
• trailing whole-block remove: O(BlocksInfo metadata) plus truncation

Untouched existing storage blocks are never read through the modifier and never rewritten. Front-loaded BlocksInfo remains rebuild-only because growing that metadata shifts the data stream by construction.

Tests

Added descriptor-level I/O-budget coverage using a 4 MiB untouched stored node. A tiny tail add and removal of the final independent block must stay below 128 KiB of archive reads/writes while the 4 MiB block remains at the same physical offset and extracts byte-identically. Zero-length removal is likewise metadata-only, and a shared-block removal is required to fall back rather than corrupt a survivor.

This PR is reconstructed on current main from the previously prepared branch; the two modified pre-existing files were byte-identical upstream, so the resulting diff contains only the intended four UnityFS files.