docs: mark boot-substrate bind DONE (systemd units, survives power cycle); update open items

STARTUP-README.md

@@ -51,10 +51,13 @@ is real. Each must be reproduced live, not asserted.
    the durable daemon via `cubec --socket /run/cube/cube.sock 'open ...'`.
    Verified: dispatch reaches the crypto/run layer. **STATUS: DONE.**
 4. **Boot substrate** — the VM brings up CUBELinux as a storage layer the rest of
-   the OS reads/writes through. **PARTIAL 2026-08-13**: OS state (boot manifest +
-   kernel.log) now lives on the cube store at `/cubefs/c200/z002/y001/x001` etc.,
-   read back with inode == packed CZYX. Still auxiliary `/cubefs` (not yet the
-   root fs) — deepening the bind is the active task. **STATUS: IN PROGRESS.**
+   the OS reads/writes through. **DONE (systemd-managed, 2026-08-13)**: units
+   `cube-os-state.service` (oneshot, writes boot manifest + machine identity +
+   boot history to `/cubefs/c200/z001|z002|z003`) and `cube-os-klog.service`
+   (streams kernel ring buffer to `/cubefs/c200/z004`) run at boot; both enabled
+   + active; state survives a FULL power cycle (verified 2026-08-13: rebooted the
+   VM, boot history showed pre- + post-reboot lines, manifest unchanged).
+   **STATUS: DONE (auxiliary layer; not yet the root fs).**
 
 ---
 
@@ -111,13 +114,15 @@ missing.
 
 ## 5. Open items
 
-- Phase 2/3: **deepen the boot-substrate bind** — make init mount the cube store
-  as a primary OS layer (e.g. bind `/var/log`, the boot manifest, and a config
-  tree to `/cubefs/c200/...` so the OS reads/writes its state through the cube by
-  default), and add each binding as a reproducible verification point above.
 - Phase 3 (optional hardening): the `cube open` primitive currently needs a key
   cell present; wire a real key-management flow (Null-space key cells issued at
   boot) so OS services can open records by CZYX + flags unattended.
+- Boot substrate (next deepening): bind a real OS tree (e.g. `/var/log`,
+  `/etc` config, the boot manifest) so the OS reads/writes through the cube by
+  default — currently the cube holds OS *state* (manifest/identity/klog) but the
+  root fs is still ext4. The magic-prefix "open by CZYX" FUSE passthrough is the
+  natural next step (a path like `/cubefs/.czyx/200.1.1.1` resolves directly to
+  the coordinate).
 - OPTIONAL (cosmetic): launch the "CUBE Shell" desktop launcher in the VM to
   confirm end-to-end at the file level (wiring already verified:
   cube-term → cubec → /run/cube/cube.sock).