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/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" {