Upload shopper registrations, and charge GST to the lines that earned it

Two defects that share a shape: a figure landing on the wrong record.

Bill-level discounts were apportioned across every line by a single
factor, so "20% off Beverages" pulled tax out of the atta line as well.
The bill total was right either way, which is what made it easy to ship
— only the slab split on a filed return was wrong. Targeted campaigns
now reduce the lines they name, and bill-wide reductions still spread
pro rata, so the arithmetic is unchanged wherever it was already right.

Shoppers registered at a till only ever reached the back office as three
fields riding along on a bill. Somebody who signed up and bought nothing
existed on one terminal and nowhere else, and two tills registering the
same mobile each minted their own row. Customers are now an outbox of
their own on pos/{store}/{terminal}/customer, and the id is a UUIDv5
over the normalised mobile number — so a hundred terminals agree on who
a shopper is without talking to each other.

Registrations go up before bills, and a failure there cannot strand a
day's takings. No loyalty figures are sent: they belong to the bill
stream, which is idempotent and knows about every counter.

Two things found while building it. Numbers were keyed on raw digits, so
a cashier typing +91 forked a shopper as effectively as a random id
would. And the sale path wrote the customer with ConflictAlgorithm
.replace, which is a DELETE and an INSERT — every column absent from the
row reverts to its schema default, so the new sync flag would have been
cleared by the shopper's next purchase.

Schema v8. Existing customers are queued rather than assumed sent: the
terminal cannot tell an imported row from a locally registered one, and
only one of those mistakes loses somebody.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/domain/entities/promo.dart

@@ -1,6 +1,7 @@
 import 'package:equatable/equatable.dart';
 
 import '../../core/constants/app_constants.dart';
+import 'product.dart';
 
 /// What a promo does to a bill.
 enum PromoType {
@@ -115,6 +116,28 @@ class Promo extends Equatable {
   static DateTime _endOfDay(DateTime day) =>
       DateTime(day.year, day.month, day.day, 23, 59, 59, 999);
 
+  /// Whether this campaign is aimed at [product] in particular.
+  ///
+  /// A bill-wide promo targets everything; a category or product one targets
+  /// only what it names. Two things read this and they must never disagree:
+  /// `PromoEngine` uses it to price the discount, and [Cart] uses it to decide
+  /// which lines carry the GST reduction. A campaign priced against one set of
+  /// lines and taxed against another puts the wrong figure in a slab on a
+  /// filed return, so the rule lives here once rather than in both callers.
+  bool targets(Product product) => switch (type) {
+        PromoType.percentOffBill || PromoType.flatOffBill => true,
+
+        // Matched on the enum name, which is stable across a label change —
+        // renaming "Personal Care" must not silently switch off a campaign.
+        PromoType.percentOffCategory => product.category.name == targetId,
+
+        PromoType.percentOffProduct || PromoType.buyXGetY =>
+          product.id == targetId,
+      };
+
+  /// Whether the discount lands on named lines rather than the whole bill.
+  bool get isTargeted => type.needsTarget;
+
   /// One-line description for the campaign list.
   String get summary => switch (type) {
         PromoType.percentOffBill => '${_trim(value)}% off the whole bill',