ordfs: serve ordfs/dir manifests and ordfs/patch vcdiff

[shruggr]

HTTP gateway support for the new ordfs/dir and ordfs/patch content types.

• Parse/validate binary directory manifests; walk them like ord-fs/json (mixed trees ok). KIND/EXEC/SYMLINK flags are ignored.
• Apply vcdiff patch chains on the way out (max 8), inheriting the base content type. A patch of a directory can be path-walked.
• ?raw returns the envelope (dir bytes or patch record). Load() is unchanged.

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