cart: back projection cache with platform/catalog.ProjectionStore

Consumer side of the catalog-projection seam (C1):
- catalogProjectionCache now wraps ProjectionStore[catalog.Projection] (identity
  transform — the cart's overlay uses ~all fields); delivery handler routes bus
  frames via HandleFrame (snapshot begin|chunk|end + epoch-stamped deltas).
- Per-pod cold-start-ready .bin under CART_PROJECTION_DIR (pod-local, never
  shared) + IsDeleted tombstone passthrough; HTTP fetch demoted to last-resort.
- Tests: test-only Apply shim + mustCache helper keep existing cases green;
  new snapshot-framing integration test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/cart/projection_store_integration_test.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"strconv"
+	"testing"
+
+	"git.k6n.net/mats/platform/catalog"
+	"git.k6n.net/mats/platform/money"
+)
+
+// frameMeta builds the event.Meta a producer stamps for a snapshot frame / delta.
+func frameMeta(phase string, epoch int64) map[string]string {
+	m := map[string]string{catalog.MetaEpoch: strconv.FormatInt(epoch, 10), catalog.MetaTenant: "se"}
+	if phase != "" {
+		m[catalog.MetaSnapshotPhase] = phase
+	}
+	return m
+}
+
+func enc(t *testing.T, ups []catalog.ProjectionUpdate) []byte {
+	t.Helper()
+	b, err := catalog.EncodeUpdates(ups)
+	if err != nil {
+		t.Fatalf("encode: %v", err)
+	}
+	return b
+}
+
+// TestCartCache_SnapshotThenDelta exercises the cart's cache through the real bus
+// framing: a full begin→chunk→end snapshot makes SKUs Get-able cold (no Apply),
+// then an epoch-matched delta overrides one and tombstones another.
+func TestCartCache_SnapshotThenDelta(t *testing.T) {
+	c := mustCache(t)
+
+	const epoch = int64(1000)
+	if err := c.HandleFrame(frameMeta(catalog.SnapshotBegin, epoch), enc(t, nil)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.HandleFrame(frameMeta(catalog.SnapshotChunk, epoch), enc(t, []catalog.ProjectionUpdate{
+		{Projection: catalog.Projection{ID: "sku:A", SKU: "A", PriceIncVat: money.Cents(100_00), TaxClass: "standard", ItemID: 1}},
+		{Projection: catalog.Projection{ID: "sku:B", SKU: "B", PriceIncVat: money.Cents(50_00), TaxClass: "reduced", ItemID: 2}},
+	})); err != nil {
+		t.Fatal(err)
+	}
+	// end carries the total count for the completeness guard.
+	endMeta := frameMeta(catalog.SnapshotEnd, epoch)
+	endMeta[catalog.MetaCount] = "2"
+	if err := c.HandleFrame(endMeta, enc(t, nil)); err != nil {
+		t.Fatal(err)
+	}
+
+	if got, ok := c.Get("A"); !ok || got.PriceIncVat != 100_00 {
+		t.Fatalf("A from snapshot = %+v ok=%v, want 10000", got, ok)
+	}
+
+	// Epoch-matched delta: bump A's price, delete B.
+	if err := c.HandleFrame(frameMeta("", epoch), enc(t, []catalog.ProjectionUpdate{
+		{Projection: catalog.Projection{SKU: "A", PriceIncVat: money.Cents(90_00), TaxClass: "standard", ItemID: 1}},
+		{Projection: catalog.Projection{SKU: "B"}, Deleted: true},
+	})); err != nil {
+		t.Fatal(err)
+	}
+	if got, _ := c.Get("A"); got.PriceIncVat != 90_00 {
+		t.Fatalf("A after delta = %d, want 9000", got.PriceIncVat)
+	}
+	if _, ok := c.Get("B"); ok {
+		t.Fatalf("B after delete should miss")
+	}
+	if !c.IsDeleted("B") {
+		t.Fatalf("B should be tombstoned (skip HTTP fetch)")
+	}
+}