refactor(props): key active fields by a typed enum, not a sentinel string

The \\\x01attr\\x01…\ sentinel-prefixed string was used to tell geometry
fields (\&'static str\ names) apart from block attribute rows (runtime
tags) in \ctive_field\ and \dit_buf\. Replace it with a \FieldKey\
enum (\Geom(&'static str)\ / \Attr(String)\), so the two namespaces can't
collide by construction: \dit_buf\ is now \HashMap<FieldKey, String>\,
\ctive_field\ is \Option<FieldKey>\, and the sentinel decoding in
\ctive_key_id\ is replaced by \FieldKey::widget_id\.

While touching the key plumbing, replace \prop_field_key_for_id\'s
per-event scan of every section x prop (with a \String\ format per
candidate) with an O(1) precomputed \HashMap<iced::widget::Id, FieldKey>\
(\ield_key_by_id\), built once in \
efresh_properties\ alongside the
sections — and rebuilt after \make_sections_read_only\ so rows demoted
to read-only stop mapping. \PropSyncActive\ now does a single map lookup
per event.

src/app/properties.rs

@@ -1432,6 +1432,9 @@ impl OpenCADStudio {
                     }
                 }
             };
+            // Precompute the focused-id → field-key map for O(1) lookups on
+            // `PropSyncActive`; derived from `sections`, so rebuild it here.
+            panel.field_key_by_id = crate::ui::properties::build_field_key_map(&panel.sections);
             panel.color_palette_open = color_palette_open;
             let new_handles: Vec<acadrust::Handle> = selected.iter().map(|(h, _)| *h).collect();
             // Carry the in-progress edits only when the selection is unchanged
@@ -1466,6 +1469,10 @@ impl OpenCADStudio {
                     .all(|handle| self.tabs[i].scene.is_layer_locked(*handle));
             if locked_only {
                 make_sections_read_only(&mut panel.sections);
+                // Rows demoted to read-only no longer back an editable field;
+                // drop them from the id→key map so focus can't map onto them.
+                panel.field_key_by_id =
+                    crate::ui::properties::build_field_key_map(&panel.sections);
                 panel.edit_buf.clear();
                 panel.active_field = None;
                 panel.color_picker_open = false;

src/app/update/command.rs

@@ -2005,7 +2005,10 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
                         // selected inserts to the picked definition. A stale
                         // typed value in the text buffer would mask the pick,
                         // so drop it; the pick also closes the list.
-                        self.tabs[i].properties.edit_buf.remove("block");
+                        self.tabs[i]
+                            .properties
+                            .edit_buf
+                            .remove(&crate::ui::properties::FieldKey::Geom("block"));
                         self.tabs[i].properties.edit_choice_open = false;
                         let canon = self.tabs[i]
                             .scene
@@ -2144,7 +2147,11 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
                 self.tabs[i].properties.active_field = None;
                 let handles = self.property_target_handles(i);
                 if !handles.is_empty() {
-                    if let Some(raw_val) = self.tabs[i].properties.edit_buf.remove(field) {
+                    if let Some(raw_val) = self.tabs[i]
+                        .properties
+                        .edit_buf
+                        .remove(&crate::ui::properties::FieldKey::Geom(field))
+                    {
                         // Block names are free-form text — a name like "10-5"
                         // must not be arithmetic-evaluated.
                         let val = if field == "block" {

src/app/update/mod.rs

@@ -4598,7 +4598,7 @@ impl OpenCADStudio {
                 self.tabs[self.active_tab]
                     .properties
                     .edit_buf
-                    .insert(field.to_string(), value);
+                    .insert(crate::ui::properties::FieldKey::Geom(field), value);
                 Task::none()
             }
 
@@ -4642,11 +4642,8 @@ impl OpenCADStudio {
             Message::PropSyncActive(focused) => {
                 let panel = &mut self.tabs[self.active_tab].properties;
                 if let Some(id) = focused.as_ref() {
-                    if let Some(key) = crate::ui::properties::prop_field_key_for_id(
-                        &panel.sections,
-                        id,
-                    ) {
-                        let changed = panel.active_field.as_deref() != Some(key.as_str());
+                    if let Some(key) = panel.prop_field_key_for_id(id) {
+                        let changed = panel.active_field.as_ref() != Some(&key);
                         panel.active_field = Some(key);
                         // Only select the whole value when focus landed on a
                         // field that wasn't already the active one; re-focusing
@@ -4659,7 +4656,7 @@ impl OpenCADStudio {
                     }
                 }
                 if !crate::ui::properties::active_key_focused(
-                    panel.active_field.as_deref(),
+                    panel.active_field.as_ref(),
                     focused.as_ref(),
                 ) {
                     panel.active_field = None;