refactor: move item groups to generic factories, move inline itemgroup loading to generic readers

[WishDuck]

Purpose of change (The Why)

Item groups defied every law of the universe
Only item_group_id::is_valid was implemented, and it was rarely used
Item groups were shoved into item factories
Item groups could not copy from each other

#9755 showed that there was issues with inline itemgroup loading.
But failed to recognize that this also applies to professions, monster weapons, construction, vehicle bash results and mapdata locations. And instead implemented a single solution instead of an overarching one.
It also failed to support deleting specific things / references from monsters if neccesary

Describe the solution (The How)

1. Move item groups to generic factories, adding required default copy from and reset variable support to generic factories

2. Made optional return a bool if a value was applied, as itemgroup loading depended on that logic and it would be useful elsewhere

3. Shift item groups from using unique pointers to shared pointers, as item group modification is limited to adding and removing these pointers, thus it is compatible.

4. Add itemgroup_reader generic reader for usage in loading inline itemgroups. It supports copy from along with all standard methods of itemgroup inheritance

5. revert feat: make death drops extendable #9755 as it is not necessary to destandardize from default itemgroup stuff and this allows default extend and standard itemgroup delete by default ( I am willing to budge on this, but there is a caviat, it will stick out as a completely different way to define nested itemgroups )

Describe alternatives you've considered

Split this into two PRs
Keep most of the old loading as halfway through the refactor I found moving from unique to shared pointers was enough

Testing

It loads the C.R.I.T. extend and delete stuff right
New tests pass which show all the inline inheritance stuff working ( individual types do not need to be tested as they all use the same runner )

Checklist

Mandatory

• I linked any relevant issues using github keyword syntax like closes #1234 in Summary of the PR so it can be closed automatically.

Optional

• This is a C++ PR that modifies JSON loading or behavior.
  • I have documented the changes in the appropriate location in the docs/ folder.

Translations

post	changed	stale	missing
mod/json/reference/items/item_spawn.md	en	ja, ko

Build Artifacts

PR build for commit 4c2dd14 (fix inline defaults and cleanup) on 2026-07-04 02:41:04

• ✅ Linux tiles — download
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• ✅ macOS tiles — download
• ✅ Android tiles — download

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