feat(userreg): prompt USERI/USERR for register then value

USERI/USERR set one of five integer/real header registers and needed both
inputs (register + value) typed on one line; bare printed usage. Add a
two-step front-end (UserRegCommand): it prompts for the register as 1–5
buttons, then the value, and dispatches USERI/USERR <n> <value> to the
existing inline handler. The argument arm now requires a space so the bare
name reaches the stepped command.

src/app/commands/view.rs

@@ -135,7 +135,14 @@ impl OpenCADStudio {
             // even though we don't have a LISP / DIESEL runtime yet.
             //   USERI 1 42        → header.user_int1 = 42
             //   USERR 3 1.5e-3    → header.user_real3 = 0.0015
-            cmd if cmd.starts_with("USERI") || cmd.starts_with("USERR") => {
+            "USERI" | "USERR" => {
+                use crate::command::UserRegCommand;
+                let name = if cmd == "USERR" { "USERR" } else { "USERI" };
+                let c = UserRegCommand::new(name);
+                self.command_line.push_info(&c.prompt());
+                self.tabs[i].active_cmd = Some(Box::new(c));
+            }
+            cmd if cmd.starts_with("USERI ") || cmd.starts_with("USERR ") => {
                 let is_real = cmd.starts_with("USERR");
                 let rest = if is_real {
                     cmd.trim_start_matches("USERR").trim()

src/command.rs

@@ -242,6 +242,77 @@ impl CadCommand for RenameCommand {
     }
 }
 
+/// Interactive front-end for USERI / USERR — the five integer / real user
+/// registers in the drawing header. They take two inputs (which register, then
+/// the value), so they prompt for the register as clickable `1`–`5` buttons and
+/// then the value, and delegate to the inline `USERI <n> <value>` handler.
+pub struct UserRegCommand {
+    /// `"USERI"` or `"USERR"`.
+    name: &'static str,
+    /// Chosen register 1–5 once the first step is answered.
+    slot: Option<u8>,
+}
+
+impl UserRegCommand {
+    pub fn new(name: &'static str) -> Self {
+        Self { name, slot: None }
+    }
+}
+
+impl CadCommand for UserRegCommand {
+    fn name(&self) -> &'static str {
+        self.name
+    }
+
+    fn prompt(&self) -> String {
+        match self.slot {
+            None => format!("{}  which register?  [1-5]:", self.name),
+            Some(n) => format!("{}{n}  new value:", self.name),
+        }
+    }
+
+    fn options(&self) -> Vec<CmdOption> {
+        match self.slot {
+            None => (1..=5).map(|n| CmdOption::new(&n.to_string(), &n.to_string())).collect(),
+            Some(_) => Vec::new(),
+        }
+    }
+
+    fn wants_text_input(&self) -> bool {
+        true
+    }
+
+    fn on_text_input(&mut self, text: &str) -> Option<CmdResult> {
+        let t = text.trim();
+        if t.is_empty() {
+            return None;
+        }
+        match self.slot {
+            None => {
+                // Accept a register 1–5; re-prompt on anything else.
+                match t.parse::<u8>() {
+                    Ok(n @ 1..=5) => {
+                        self.slot = Some(n);
+                        None
+                    }
+                    _ => None,
+                }
+            }
+            // The inline handler validates the value (int vs real) and reports
+            // usage if it doesn't parse, so just hand the whole line over.
+            Some(n) => Some(CmdResult::Dispatch(format!("{} {n} {t}", self.name))),
+        }
+    }
+
+    fn on_point(&mut self, _pt: DVec3) -> CmdResult {
+        CmdResult::NeedPoint
+    }
+
+    fn on_enter(&mut self) -> CmdResult {
+        CmdResult::Cancel
+    }
+}
+
 // ── Result token ──────────────────────────────────────────────────────────
 
 /// Returned by every `CadCommand` method to tell main.rs what to do.