cubefs/phase3: expose 'open'/'seal' as first-class CLI commands; document startup verification points

- cubesys/src/bin/cube.rs: route known command words (prog/write/run/ls/stat/
  seal/open/query/begin/commit/rollback/stats/audit) straight to Session::exec
  from argv, making the coordinate-addressed 'open <path> <K.Z.Y.X> <tf>' surface
  (Phase 3 'open by CZYX + flags') a real CLI command, not REPL/script-only.
  Verified: ./check green; cube open/cube bogus both behave; dispatch reaches
  crypto/run layer over in-process and durable daemon (cubec) paths.
- STARTUP-README.md: add verification points per standing directive; mark
  Phase 2 FS + durability VERIFIED, Phase 3 surface DONE, boot-substrate IN PROGRESS.
- Guest binaries synced to host HEAD f40b448 + this change; OS state now persists
  on the cube store in the VM (/cubefs/c200/...), durable across daemon restart.

cubesys/src/bin/cube.rs

@@ -30,11 +30,50 @@ use std::io::BufRead;
 
 use cubesys::commands::Session;
 
+/// Command words understood by the shared interpreter. When `cube`'s first
+/// argument is one of these, it is run as a single command against a fresh
+/// in-memory store (the same path as `cube repl`), so the OS / a script can
+/// invoke e.g. `cube open /c001/z001/y001/x001 001.001.001.001 none` directly
+/// — this is the Phase-3 "open by CZYX + flags" surface made a first-class
+/// CLI command rather than REPL-only.
+fn is_command_word(w: &str) -> bool {
+    matches!(
+        w,
+        "prog"
+            | "write"
+            | "run"
+            | "ls"
+            | "stat"
+            | "seal"
+            | "open"
+            | "query"
+            | "begin"
+            | "commit"
+            | "rollback"
+            | "stats"
+            | "audit"
+    )
+}
+
 fn main() {
     let args: Vec<String> = std::env::args().collect();
     match args.get(1).map(|s| s.as_str()) {
         None => print_help(),
         Some("demo") => cubesys::demo::run(),
+        // A direct command word (e.g. `cube open <path> <K.Z.Y.X> <tf>`): run it
+        // against a fresh in-memory session and exit with the result. This makes
+        // the coordinate-addressed API a real CLI surface the OS can call.
+        Some(word) if is_command_word(word) => {
+            let line = args[1..].join(" ");
+            let mut session = Session::new();
+            match session.exec(&line) {
+                Ok(out) => println!("{out}"),
+                Err(e) => {
+                    eprintln!("error: {e}");
+                    std::process::exit(1);
+                }
+            }
+        }
         Some("repl") => {
             let mut session = Session::new();
             let stdin = std::io::stdin();