Xref attach: Attach dialog, reference prompts, and xref drawings other applications can open

Attach External Reference dialog after XATTACH / ATTACH; ATTACH routes
drawings, PDFs and images; placement prompts and messages follow the
reference (presets, Corner/XYZ, nonzero check, existing definition reuse,
missing file, self-reference); name keeps the file's case; overlay and
path type at attach; one undo step for definition and INSERT; -XREF option
prompt and `?` table; XREF opens the palette; ER alias; xref Properties
rows; dependent symbols flagged and remapped and resolved content left out
of the saved file; dialog_ok automation action; localized labels.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/commands/mod.rs

@@ -5,6 +5,7 @@ use iced::Task;
 use std::path::PathBuf;
 
 mod blocks;
+mod xref_attach;
 mod dim;
 pub(crate) mod display;
 mod draw;
@@ -246,6 +247,9 @@ impl OpenCADStudio {
         if let Some(t) = self.dispatch_layers(cmd, i) {
             return Some(t);
         }
+        if let Some(t) = self.dispatch_xref_attach(cmd, i) {
+            return Some(t);
+        }
         if let Some(t) = self.dispatch_blocks(cmd, i) {
             return Some(t);
         }
@@ -652,6 +656,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "3DORBIT",
         "3O",
         "ABOUT",
+        "ATTACH",
         "ATTDISP",
         "ATTEXT",
         "BACKGROUND",