Overlay only the fields a menu extension declares

[vovarbv]

docs/menu.md and the shipped sample extension both promise that reusing a shipped id replaces only the fields you declare, so an extension can retitle or re-icon a row without re-declaring its action. normalizeItem materializes every field with a default before mergeMenuSources overlays the entry, so a one-field override carried the other thirteen defaults with it.

The documented example is enough to reproduce it. With ~/.config/omarchy/extensions/omarchy-menu.jsonc containing {"items": {"about": {"icon": "X"}}}:

shipped   label="About" kind=action action="omarchy-launch-about" icon=""   visible=true
before    label="about" kind=menu   action=""                     icon="X"  visible=false
after     label="About" kind=action action="omarchy-launch-about" icon="X"  visible=true

The row lost its action, its label fell back to the bare id, it became a childless submenu, and isVisible then dropped it from the menu entirely.

normalizeItem now records which keys the author wrote and mergeMenuSources overlays only those, re-deriving kind from whatever action or target survived. A new id declared only in the extension is still taken whole, and merging with no extension leaves the shipped tree identical.

mergeAppRows and swapProviderRows replace whole rows rather than overlaying fields, so they were not affected.

The existing menu normalizes parsed items assertion pins the whole normalized item, so it gains the new key. The added assertions fail on quattro and pass with the change; ./test/shell and ./test/cli show no other difference from the branch point.