4. Writing the filesystem - done, all three layers

A package nests two filesystems and they are built to different rules. Both are written now, and so is the container between them:

• selfish_pfs::write - the inner image: superblock, inode table, directory entries, block allocation, and the flat path table's real hash index. Plain, because that is what the inner image is - PfsProperties.cs builds it unsigned and unencrypted and three real images agree. (D049, D057)
• selfish_pfs::pfsc - the container. It does not compress: the block map is absolute offsets and a full-size block is stored as-is, which was on the blocker list under the wrong description for a long time. (D050)
• selfish_pfs::outer - the outer image: signed and encrypted, holding the container as its single file. Every key comes from the content id and the passcode; nothing has to be recovered. (D052, D053)

The 32 bytes at 0x380 are no longer unexplained. They are the header's own block signature - an HMAC-SHA256 under a key derived from EKPFS, followed by the block index, in the same 36-byte shape as every other block signature in the image. LibOrbisPkg writes them as BlockSigInfo(0, 0x380, 0x5A0). The measurement that called them "33 non-zero bytes of something" was counting one signature and the first byte of the index after it. (D052)

Still open here: a payload needing a doubly-indirect signature block (past roughly 117 MiB) returns an error rather than a guessed layout, and a collision resolver for two paths that hash alike is not written - has_collision answers the question before building and build refuses rather than dropping an entry. (D053, D057)