Function filesystem_open 

pub fn filesystem_open<R: Read + Seek>(reader: R) -> Result<OpenFilesystem<R>>

Open a btrfs filesystem by bootstrapping from the superblock.

This performs the full bootstrap sequence:

1. Read the superblock (mirror 0)
2. Seed the chunk cache from the sys_chunk_array
3. Read the full chunk tree to complete the cache
4. Read the root tree to collect all tree root pointers

Errors

Returns an error if any I/O operation fails during bootstrap.