feat(model): use IndexMap for deterministic insertion order in DataStore

Replace `HashMap` with `IndexMap` in `DataStore::cells` to preserve
insertion order. This allows records mode to display rows in the order they
were added. Update `remove` to use `shift_remove` to maintain order during
deletions.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-26B-A4B-it-UD-Q5_K_XL.gguf)

src/model/cell.rs

@@ -1,3 +1,4 @@
+use indexmap::IndexMap;
 use serde::{Deserialize, Serialize};
 use std::collections::{HashMap, HashSet};
 
@@ -104,8 +105,9 @@ pub struct InternedKey(pub Vec<(Symbol, Symbol)>);
 /// to implement the `Serialize`-as-string requirement for JSON object keys.
 #[derive(Debug, Clone, Default)]
 pub struct DataStore {
-    /// Primary storage — interned keys for O(1) hash/compare.
-    cells: HashMap<InternedKey, CellValue>,
+    /// Primary storage — interned keys, insertion-ordered so records mode
+    /// can display rows in the order they were entered.
+    cells: IndexMap<InternedKey, CellValue>,
     /// String interner — all category/item names are interned here.
     pub symbols: SymbolTable,
     /// Secondary index: interned (category, item) → set of interned keys.
@@ -193,7 +195,7 @@ impl DataStore {
         let Some(ikey) = self.lookup_key(key) else {
             return;
         };
-        if self.cells.remove(&ikey).is_some() {
+        if self.cells.shift_remove(&ikey).is_some() {
             for pair in &ikey.0 {
                 if let Some(set) = self.index.get_mut(pair) {
                     set.remove(&ikey);