Add unit tests for model precision and category tree rendering:
- `src/model/types.rs` : Added `formula_chain_preserves_full_precision` to
ensure formulas use full `f64` precision for calculations, even when
display is rounded.
- `src/ui/cat_tree.rs` : Added comprehensive tests for `build_cat_tree` ,
covering empty models (virtual categories), expanded/collapsed states,
and item rendering.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)
Add a tree-based category panel that supports expand/collapse of categories.
Introduces CatTreeEntry and build_cat_tree to render categories as
a collapsible tree. The category panel now displays categories with
expand indicators (▶/▼) and shows items under expanded categories.
CmdContext gains cat_tree_entry(), cat_at_cursor(), and cat_tree_len()
methods to work with the tree. App tracks expanded_cats in a HashSet.
Keymap updates: Enter in category panel now triggers filter-to-item.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)
