feat(cubelinux-2): Package 3 — cubefs (POSIX/FUSE namespace over CZYX)

Implements the PDF's Package 3 with new code:

- path: bijective POSIX path <-> Czyx mapping (/c001/z002/y003/x004).
  Axis-letter + 3-digit zero-padded canonical names so lexical order equals
  numeric order and each coordinate has exactly one spelling. Inode IS the
  packed u32 coordinate — no inode side table.
- nullspace: the PDF's 'use Null cubes for ACLs, xattrs, journaling, volume
  metadata', with the Z-plane allocation fixed and documented (Z=1 volume,
  Z=2 ACL, Z=3 xattr, Z=4 journal ring). ACL/xattr tables are FNV
  hash-bucketed with exact-match resolution inside the bucket, because 4
  axes of subject cannot injectively mirror into 2 axes of Null space.
  Journal is a bounded ring; wraps are detectable via a monotonic counter.
- vfs: the whole filesystem, kernel-free and unit-testable — lookup,
  readdir, create/read/write/truncate/unlink, mkdir/rmdir, ACL enforcement,
  xattrs, journaling, POSIX errno mapping.
- fuse (feature 'mount'): thin kernel adapter, zero TTL (the store is
  writable out-of-band, so cached metadata would go stale).
- cubestore: added the PDF's 'optional scanning primitives' (keys,
  scan_prefix) and the Package 2 association API (associate, linked_to)
  that cubefs needs for directory listings.

Two defects were found by LIVE MOUNT testing and fixed, not by unit tests:
 1. mkdir succeeded then the kernel's revalidating lookup returned ENOENT,
    so 'mkdir -p' could never reach depth 4. Directories were purely
    inferred from records, making an empty directory unrepresentable. Fixed
    with an explicit Null-space directory marker; rmdir removes it; readdir
    merges markers in. 5 regression tests added.
 2. Multi-user ACL behaviour was untestable because the mount lacked
    AllowOther — the kernel returned EACCES at the mountpoint before any
    request reached us. Added --allow-other.

Verified: 58 unit tests pass; clippy clean; live mount exercised with cat,
echo, dd, truncate, cp, chmod, chown, getfattr/setfattr, mkdir -p, rmdir,
find, a 200-record write loop, and cross-user reads/writes as luulu.

cubecoords/src/lib.rs

@@ -52,10 +52,7 @@ impl Czyx {
     /// Pack into a single `u32` with `C` in the high byte, `X` in the low byte.
     #[inline]
     pub fn pack_u32(&self) -> u32 {
-        ((self.c as u32) << 24)
-            | ((self.z as u32) << 16)
-            | ((self.y as u32) << 8)
-            | (self.x as u32)
+        ((self.c as u32) << 24) | ((self.z as u32) << 16) | ((self.y as u32) << 8) | (self.x as u32)
     }
 
     /// Unpack a `u32` produced by [`pack_u32`].