pub const FEATURE_FS: u32 = _; // 64u32Expand description
S2C_HELLO feature bit: server supports the FS_* message family,
reads and writes alike (docs/design/fs-watch.md, docs/design/fs-write.md).
A read-only deployment (BLIT_FS_WRITE=0) still advertises this bit and
answers FS_WRITE/FS_OP with FS_DONE_PERMISSION.