Feature: Creating Cart Views, rules for abandonment

docs/cart.md

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+# Cart Admin Visibility
+
+`Modules\Core\Cart\Filament\Resources\CartResource` gives staff read-only visibility into
+customer/user carts in the Filament admin panel. Lunar itself ships no cart admin view at
+all — no Filament resource for `Cart`/`CartLine` exists anywhere in `lunarphp/lunar` or
+`lunarphp/core` — this is a from-scratch addition, not an extension of something Lunar
+half-built. See `docs/lunar.md`'s "Cart and Checkout" section for the underlying Lunar cart
+mechanics this resource reads from.
+
+---
+
+## Scope: only carts with a known customer or user
+
+`CartResource::getEloquentQuery()` filters to `Cart::whereNotNull('user_id')->orWhereNotNull('customer_id')`
+— an anonymous guest's session cart is excluded entirely.
+
+This was a deliberate call, not an oversight: an anonymous cart carries no identity a staff
+member could act on — no name, no email, nothing to follow up with — so listing every guest
+session cart would be noise, not a real admin capability. This does **not** mirror Shopify's
+admin (Shopify has no "all carts" view at all — only "Abandoned checkouts," gated on a
+shopper reaching checkout and entering contact info, a later/narrower stage than Lunar's
+`Cart`). Lunar's own `Cart` model already gets `user_id`/`customer_id` set the moment a
+shopper is authenticated (via `Lunar\Listeners\CartSessionAuthListener` on login), with no
+checkout step required — so scoping to "identifiable" here is broader than Shopify's
+equivalent, not a copy of it.
+
+---
+
+## "Abandoned" vs "Completed" — not `Cart::completed_at`
+
+`Lunar\Models\Cart::completed_at` is declared and cast (`'completed_at' => 'datetime'`) but
+**never actually written anywhere in Lunar core** — grep `vendor/lunarphp/core/src` for it;
+the only hits are the property declaration and the cast. It is not a real signal.
+
+The list page's tabs (`ListCarts::getTabs()`) instead key off whether the cart has a
+**placed** order:
+
+- **Abandoned** — mirrors `Cart::scopeActive()` exactly: no orders at all, or only orders
+  still in draft (`placed_at IS NULL`). Default active tab on page load.
+- **Completed** — has at least one order with `placed_at IS NOT NULL`.
+
+```php
+// Abandoned
+$query->active();
+
+// Completed
+$query->whereHas('orders', fn ($query) => $query->whereNotNull('placed_at'));
+```
+
+There is deliberately **no "All" tab.** Every row shown is always scoped to one of the two
+states above — the list never runs an unfiltered `Cart::query()->get()` over the whole
+(potentially large) table.
+
+---
+
+## Why this scales fine at a large cart count
+
+Two things keep this cheap regardless of how many carts exist (10,000+):
+
+- **The list is always paginated.** Filament applies `LIMIT`/`OFFSET` to whichever tab's
+  query is active — a page only ever fetches one page's worth of rows, never the whole
+  table, "All" tab or not (and there is no "All" tab — see above).
+- **No per-row queries.** `lines_count`/`lines_sum_quantity` use Filament's built-in
+  `->counts('lines')`/`->sum('lines', 'quantity')`, which fold into the same query as the
+  rest of the list (one `LEFT JOIN`-based aggregate, not N separate lookups). There's no
+  per-record `getStateUsing()` closure anywhere in this table doing its own query — that's
+  the pattern to avoid if a future column needs derived data (see `Modules\Core\Catalog\
+  Services\ProductIndexer` for the general "compute once at index time / one aggregate
+  query, never per-row" principle this project follows elsewhere).
+
+The one thing that **does** scan more rows as the cart count grows is
+`CartResource::getNavigationBadge()` (see below) — but it's a `COUNT(*)`, not a fetch, and
+runs once per admin page load, not once per cart row.
+
+---
+
+## Navigation badge — abandoned cart count
+
+```php
+public static function getNavigationBadge(): ?string
+{
+    return (string) static::getEloquentQuery()->active()->count();
+}
+```
+
+Shows the number of abandoned carts (not all carts — a converted cart isn't something a
+staff member needs to keep noticing) next to "Carts" in the sidebar. `->count()` compiles to
+a single `SELECT COUNT(*) ...` — confirmed via query log — no rows are ever loaded just to
+render the badge.
+
+---
+
+## The view page runs the cart's full calculate pipeline — once
+
+`ViewCart::resolveRecord()` calls `$cart->calculate()` before rendering, since `CartLine`'s
+computed properties (`unitPrice`, `total`, etc.) and `Cart`'s own totals (`subTotal`, `total`,
+...) are plain public properties populated as a side effect of that pipeline — never
+persisted, so a plain Eloquent-fetched `Cart` has them all `null`/unset (see `docs/lunar.md`
+Gotchas). This only runs on the single-record view page, not per row in the list table —
+running the full 5-step pipeline for every row of a paginated list would be needless cost for
+data the list doesn't display.
+
+---
+
+## Not built: staff editing a cart
+
+The resource is deliberately read-only (`canCreate()` returns `false`, no edit page
+registered). A cart is owned by the storefront's own add/update/remove flow
+(`CartSession`/`Cart::add()`/etc.) — hand-editing cart contents from the admin panel isn't a
+supported use case here.