Add plain-language item type template documentation

docs/item-schema.md

@@ -48,6 +48,7 @@
 - Global/type-level properties are loaded from server registry in `server/app/item_catalog.py`.
 - Per-type use/update validation and message behavior are handled in `server/app/item_type_handlers.py`.
 - Client-side add/edit metadata is handled in `client/src/items/itemRegistry.ts`.
+- End-to-end add-item template: `docs/item-type-template.md`.
 
 ## Type Params
 

docs/item-type-template.md

@@ -0,0 +1,118 @@
+# Item Type Template
+
+This page is a practical, copy/paste template for adding a new item type with the current registry-based system.
+
+Use this when you want a new item type without editing one huge `if/elif` chain.
+
+## Plain-English Flow
+
+When a new item type is added, you wire it in four places:
+
+1. Server catalog (`item_catalog.py`)
+- Defines global defaults shared by all instances of that type:
+  - default title
+  - default params
+  - use sound / emit sound
+  - cooldown
+
+2. Server behavior (`item_type_handlers.py`)
+- Defines what happens when users edit params (`validate_update`).
+- Defines what happens when users press `use` (`use`).
+
+3. Shared type unions (`models.py`, protocol/state types)
+- Adds the new type name to type literals/unions so packets validate.
+
+4. Client item registry (`itemRegistry.ts`)
+- Defines add-menu order, editable properties, and optional property dropdown choices.
+
+That is enough for a first working item type.
+
+## Example: `counter`
+
+This sample item increments a number each time it is used.
+
+### 1. Server Catalog (`server/app/item_catalog.py`)
+
+```py
+ItemType = Literal["radio_station", "dice", "wheel", "clock", "counter"]
+
+ITEM_DEFINITIONS: dict[ItemType, ItemDefinition] = {
+    # ...existing...
+    "counter": ItemDefinition(
+        default_title="counter",
+        capabilities=("editable", "carryable", "deletable", "usable"),
+        use_sound=None,
+        emit_sound=None,
+        default_params={"value": 0},
+        use_cooldown_ms=1000,
+    ),
+}
+```
+
+### 2. Server Handler (`server/app/item_type_handlers.py`)
+
+```py
+def _validate_counter_update(_item: WorldItem, next_params: dict) -> dict:
+    try:
+        value = int(next_params.get("value", 0))
+    except (TypeError, ValueError) as exc:
+        raise ValueError("value must be a number.") from exc
+    if value < 0:
+        raise ValueError("value must be 0 or greater.")
+    next_params["value"] = value
+    return next_params
+
+
+def _use_counter(item: WorldItem, nickname: str, _clock_formatter: Callable[[dict], str]) -> ItemUseResult:
+    current = int(item.params.get("value", 0))
+    next_value = current + 1
+    return ItemUseResult(
+        self_message=f"{item.title}: {next_value}",
+        others_message=f"{nickname} uses {item.title}: {next_value}",
+        updated_params={**item.params, "value": next_value},
+    )
+
+
+ITEM_TYPE_HANDLERS: dict[ItemType, ItemTypeHandler] = {
+    # ...existing...
+    "counter": ItemTypeHandler(
+        validate_update=_validate_counter_update,
+        use=_use_counter,
+    ),
+}
+```
+
+### 3. Type Unions
+
+Update item-type unions/literals in:
+
+- `server/app/models.py`
+- `client/src/network/protocol.ts`
+- `client/src/state/gameState.ts`
+
+Add `"counter"` anywhere item types are enumerated.
+
+### 4. Client Registry (`client/src/items/itemRegistry.ts`)
+
+```ts
+export const ITEM_TYPE_SEQUENCE: ItemType[] = ['clock', 'counter', 'dice', 'radio_station', 'wheel'];
+
+const ITEM_TYPE_EDITABLE_PROPERTIES: Record<ItemType, string[]> = {
+  // ...existing...
+  counter: ['title', 'value'],
+};
+
+export const ITEM_TYPE_GLOBAL_PROPERTIES: Record<ItemType, Record<string, string | number | boolean>> = {
+  // ...existing...
+  counter: { useSound: 'none', emitSound: 'none', useCooldownMs: 1000 },
+};
+```
+
+No dropdown options are needed here because `value` is a numeric text field.
+
+## Checklist Before Commit
+
+1. Add/adjust server tests for `use` and `update` validation.
+2. Run `cd server && uv run --extra dev pytest`.
+3. Run `cd client && npm run lint && npm run build`.
+4. Update `docs/item-types.md` and `docs/item-schema.md` if behavior/defaults changed.

docs/item-types.md

@@ -100,6 +100,8 @@ This is behavior-focused documentation for item types and their defaults.
 
 Item types are currently code-registered on both server and client so new types are additive instead of editing one large branch.
 
+For a full copy/paste example with plain-English explanation, see `docs/item-type-template.md`.
+
 1. Server catalog: add global defaults in `server/app/item_catalog.py` (`ITEM_DEFINITIONS`).
 2. Server handlers: add `validate_update` + `use` logic in `server/app/item_type_handlers.py` and register it in `ITEM_TYPE_HANDLERS`.
 3. Server models: extend `ItemType` literals in `server/app/models.py` and any packet enums that list item types.