feat(commands): step the rest of the keyword/value commands

Convert the remaining bucket-B commands to prompt step by step instead of
requiring arguments or printing usage:

- KeywordCommand (buttons): VIEW, LINETYPE, SCALELISTEDIT, UCSICON, VSCURRENT.
- ValuePromptCommand (one value): LAYDEL (layer), ATTSYNC (block).
- TwoValuePromptCommand (new; two values): LAYMRG (source + target),
  SETVAR (variable + value, blank second reads).

Each bare form now starts the stepped front-end and delegates to the existing
inline handler; the argument form is kept and is the dispatch target. The
direct visual-style shortcuts (HIDDENLINE / XRAY / …) and UCSICON's argument
form are untouched.

COUNT is intentionally left alone: bare COUNT already reports the tally (a
useful inquiry action, not a usage line), so prompting would only add friction.

src/app/commands/layers.rs

@@ -75,7 +75,13 @@ impl OpenCADStudio {
             }
 
             // LAYDEL <name> — delete a layer and erase the objects on it.
-            cmd if cmd == "LAYDEL" || cmd.starts_with("LAYDEL ") => {
+            "LAYDEL" => {
+                use crate::command::ValuePromptCommand;
+                let c = ValuePromptCommand::new("LAYDEL", "LAYDEL  layer to delete:");
+                self.command_line.push_info(&c.prompt());
+                self.tabs[i].active_cmd = Some(Box::new(c));
+            }
+            cmd if cmd.starts_with("LAYDEL ") => {
                 let name = cmd.trim_start_matches("LAYDEL").trim();
                 if name.is_empty() {
                     self.command_line.push_info("Usage: LAYDEL <layer name>");
@@ -127,7 +133,17 @@ impl OpenCADStudio {
 
             // LAYMRG <source> <target> — move every object from <source> onto
             // <target>, then delete the emptied <source> layer.
-            cmd if cmd == "LAYMRG" || cmd.starts_with("LAYMRG ") => {
+            "LAYMRG" => {
+                use crate::command::TwoValuePromptCommand;
+                let c = TwoValuePromptCommand::new(
+                    "LAYMRG",
+                    "LAYMRG  source layer (merged away):",
+                    "LAYMRG  target layer (kept):",
+                );
+                self.command_line.push_info(&c.prompt());
+                self.tabs[i].active_cmd = Some(Box::new(c));
+            }
+            cmd if cmd.starts_with("LAYMRG ") => {
                 let rest = cmd.trim_start_matches("LAYMRG").trim();
                 let parts: Vec<&str> = rest.split_whitespace().collect();
                 if parts.len() != 2 {