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/view/mod.rs

@@ -1,7 +1,7 @@
-pub mod view;
 pub mod axis;
 pub mod layout;
+pub mod view;
 
-pub use view::View;
 pub use axis::Axis;
-pub use layout::GridLayout;
+pub use layout::{AxisEntry, GridLayout};
+pub use view::View;