cubefs: add .czyx.C.Z.Y.X FUSE magic-prefix for Phase-3 'open by CZYX' (same inode as canonical path)

STARTUP-README.md

@@ -45,11 +45,18 @@ is real. Each must be reproduced live, not asserted.
    `fixed-durable-20260813`). Re-verified 2026-08-13: OS state written to
    `/cubefs/c200/...` survived a `systemctl restart cube-server cubefs`. **STATUS: VERIFIED.**
 3. **Phase 3 syscall surface** — "open by CZYX + flags": a record addressable
-   directly by coordinate, not path. **SHIPPED 2026-08-13**: `cube open <path>
-   <K.Z.Y.X> <tf>` and `cube seal ...` are now first-class CLI commands (were
-   REPL/script-only) in `cubesys/src/bin/cube.rs`; reachable in-process AND over
-   the durable daemon via `cubec --socket /run/cube/cube.sock 'open ...'`.
-   Verified: dispatch reaches the crypto/run layer. **STATUS: DONE.**
+   directly by coordinate, not path. **SHIPPED 2026-08-13**:
+   (a) `cube open <path> <K.Z.Y.X> <tf>` and `cube seal ...` are first-class CLI
+   commands in `cubesys/src/bin/cube.rs` (reachable in-process + over the durable
+   daemon). Verified: dispatch reaches the crypto/run layer.
+   (b) **`.czyx.C.Z.Y.X` FUSE magic-prefix (2026-08-13)** — `cubefs/src/path.rs`
+   `parse_dot_czyx` + `cubefs/src/fuse.rs::lookup` intercept: a path component
+   `.czyx.200.50.1.7` resolves to the SAME inode as the canonical
+   `/cubefs/c200/z050/y001/x007` and reads/writes the real record. This is the
+   Phase-3 "open by CZYX" at the filesystem level — the kernel addresses the
+   record by coordinate. LIVE VERIFIED: stat both spellings -> identical inode
+   3358720263; `cat .czyx.200.50.1.7` returns the record content. Unit tests in
+   `path.rs` cover full/partial/rejected forms. **STATUS: DONE.**
 4. **Boot substrate** — the VM brings up CUBELinux as a storage layer the rest of
    the OS reads/writes through. **DONE (systemd-managed, 2026-08-13)**: units
    `cube-os-state.service` (oneshot, writes boot manifest + machine identity +