### Summary
A user extension that overrides a **single field** of a default menu entry
(`~/.config/omarchy/extensions/omarchy-menu.jsonc`) silently resets the rest:
`label` falls back to the id, and `provider`, `action` and `aliases` are dropped.
The JSONC docs promise the opposite ("Existing fields are kept unless overridden").

Example — stock entries:
```jsonc
"apps":  { "icon": "󰀻", "label": "Apps", "aliases": ["app","applications"], "provider": "apps" },
"about": { "icon": "", "label": "About", "action": "omarchy-launch-about" }
```

User override that only wants to swap the icon:
```jsonc
"apps":  { "icon": "🧩" },
"about": { "icon": "🛈" }
```

Expected: only `icon` changes. Actual (merged):
```
apps  -> label "apps" (lowercase), provider "", aliases []
about -> label "about"            , action ""
```
Visible effects: label loses capitalization; the Apps submenu loses its provider
and enumerates nothing; the About action stops running; search aliases disappear.

### Steps to reproduce
1. Fresh `omarchy 4.0.3-1` stock shell, default user config.
2. Create `~/.config/omarchy/extensions/omarchy-menu.jsonc` containing only
   `"apps": { "icon": "🧩" }`.
3. Open the menu (Super+Space). The Apps row renders as lowercase "apps" and
   its submenu is empty.

### Root cause
In `shell/plugins/menu/MenuModel.js`:

- `normalizeItem(id, raw)` always emits every field with a default
  (`label = value.label || id`, `provider = value.provider || ""`, `aliases: []`, …),
  so the parsed user entry is full of defaults, not only the keys that were provided.
- `mergeMenuSources(defaultItems, userItems)` then copies every key of the user
  entry over the merged item (`for (var k2 in entry) merged[k2] = entry[k2]`),
  so those invisible defaults clobber the stock entry's real
  `label`/`provider`/`action`/`aliases`.

Together the two functions make partial overrides impossible, despite the
per-key wording in `rebuildItemsFromSources` ("so the user can tweak
label/icon/action without re-declaring the whole row").

### Suggested fix
Keep merge per-key, but let `parseMenuJsonc` emit only the keys actually present
in the raw entry (plus the computed `id`/`parent`/`kind`), then run a final fill
pass after merging so brand-new user entries still get their defaults
(`label = id`, `""`). Optionally add a unit test that merges a bare
`"apps": {"icon": "X"}` overlay and asserts `label === "Apps"`,
`provider === "apps"` and `aliases` preserved.

### Environment
- omarchy 4.0.3-1
- kernel 7.2.3-arch1-3

---
Filed by big-pickle via opencode.