feat: group-aware grid rendering and hide/show item

Builds out two half-finished view features:

Group collapse:
- AxisEntry enum distinguishes GroupHeader from DataItem on grid axes
- expand_category() emits group headers and filters collapsed items
- Grid renders inline group header rows with ▼/▶ indicator
- `z` keybinding toggles collapse of nearest group above cursor

Hide/show item:
- Restore show_item() (was commented out alongside hide_item)
- Add HideItem / ShowItem commands and dispatch
- `H` keybinding hides the current row item
- `:show-item <cat> <item>` command to restore hidden items
- Restore silenced test assertions for hide/show round-trip

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/command/types.rs

@@ -1,5 +1,5 @@
-use serde::{Deserialize, Serialize};
 use crate::view::Axis;
+use serde::{Deserialize, Serialize};
 
 /// All commands that can mutate a Model.
 ///
@@ -15,7 +15,11 @@ pub enum Command {
     AddItem { category: String, item: String },
 
     /// Add an item inside a named group.
-    AddItemInGroup { category: String, item: String, group: String },
+    AddItemInGroup {
+        category: String,
+        item: String,
+        group: String,
+    },
 
     /// Set a cell value.  `coords` is a list of `[category, item]` pairs.
     SetCell {
@@ -30,10 +34,16 @@ pub enum Command {
     /// Add or replace a formula.
     /// `raw` is the full formula string, e.g. "Profit = Revenue - Cost".
     /// `target_category` names the category that owns the formula target.
-    AddFormula { raw: String, target_category: String },
+    AddFormula {
+        raw: String,
+        target_category: String,
+    },
 
     /// Remove a formula by its target name and category.
-    RemoveFormula { target: String, target_category: String },
+    RemoveFormula {
+        target: String,
+        target_category: String,
+    },
 
     /// Create a new view.
     CreateView { name: String },
@@ -53,6 +63,12 @@ pub enum Command {
     /// Toggle collapse of a group in the active view.
     ToggleGroup { category: String, group: String },
 
+    /// Hide an item in the active view.
+    HideItem { category: String, item: String },
+
+    /// Show (un-hide) an item in the active view.
+    ShowItem { category: String, item: String },
+
     /// Save the model to a file path.
     Save { path: String },
 
@@ -88,12 +104,21 @@ pub struct CommandResult {
 
 impl CommandResult {
     pub fn ok() -> Self {
-        Self { ok: true, message: None }
+        Self {
+            ok: true,
+            message: None,
+        }
     }
     pub fn ok_msg(msg: impl Into<String>) -> Self {
-        Self { ok: true, message: Some(msg.into()) }
+        Self {
+            ok: true,
+            message: Some(msg.into()),
+        }
     }
     pub fn err(msg: impl Into<String>) -> Self {
-        Self { ok: false, message: Some(msg.into()) }
+        Self {
+            ok: false,
+            message: Some(msg.into()),
+        }
     }
 }