feat(commands): interactive value prompts for PDMODE/PDSIZE/LTSCALE/CELTSCALE (F4)

Bare PDMODE / PDSIZE / LTSCALE / CELTSCALE now enter a generic
ValuePromptCommand front-end that prompts for the value and delegates to the
existing inline `<name> <value>` handler via CmdResult::Dispatch, so they
prompt step-by-step like UCS. A bare Enter reports the current value;
`PDMODE 3` still works as one typed / headless line.

Only pure single-value setting commands are converted. Dialog-opening
commands (LAYER, PURGE, DIMSTYLE, STYLE …) intentionally keep their
bare-name behaviour (open the panel), and subcommand-tree commands keep
their inline handlers — all already typable with arguments since F1.

Adds headless test value_prompt_commands_inline_args. Fourth phase of the
unified step driver.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/automation.rs

@@ -497,6 +497,27 @@ mod tests {
         );
     }
 
+    #[test]
+    fn value_prompt_commands_inline_args() {
+        // A single-value setting command entered with its value on one line
+        // drives the interactive front-end (start + value step) and applies via
+        // the inline handler. (F4)
+        let mut app = OpenCADStudio::new_for_test();
+        app.automation_op(r#"{"op":"new"}"#);
+        app.automation_op(r#"{"op":"run","cmd":"PDMODE 3"}"#);
+        app.automation_op(r#"{"op":"run","cmd":"LTSCALE 2.5"}"#);
+        let i = app.active_tab;
+        let h = &app.tabs[i].scene.document.header;
+        assert_eq!(h.point_display_mode, 3, "PDMODE 3 should set point mode");
+        assert!(
+            (h.linetype_scale - 2.5).abs() < 1e-9,
+            "LTSCALE 2.5 should set scale, got {}",
+            h.linetype_scale
+        );
+        // No command should be left dangling.
+        assert!(app.tabs[i].active_cmd.is_none(), "command must have finished");
+    }
+
     #[test]
     fn save_then_open_round_trips() {
         let mut app = OpenCADStudio::new_for_test();