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_testhelpers_test.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"testing"
+
+	"git.k6n.net/mats/platform/catalog"
+)
+
+// mustCache opens a per-pod projection cache backed by a temp dir (pod-local).
+func mustCache(t *testing.T) *catalogProjectionCache {
+	t.Helper()
+	c, err := newCatalogProjectionCache(t.TempDir())
+	if err != nil {
+		t.Fatalf("newCatalogProjectionCache: %v", err)
+	}
+	t.Cleanup(func() { _ = c.store.Close() })
+	return c
+}
+
+// Apply is a TEST-ONLY convenience that routes a delta batch through the store at
+// the initial epoch (0), reproducing the pre-ProjectionStore cache API so the
+// existing cart tests keep exercising Get/IsDeleted/Len. Production code feeds the
+// store via HandleFrame with real bus framing (snapshot + epoch-stamped deltas).
+func (c *catalogProjectionCache) Apply(updates []catalog.ProjectionUpdate) (upserts, deletes int) {
+	for _, u := range updates {
+		if u.Deleted {
+			deletes++
+		} else if u.SKU != "" {
+			upserts++
+		}
+	}
+	payload, err := catalog.EncodeUpdates(updates)
+	if err != nil {
+		return 0, 0
+	}
+	// No MetaSnapshotPhase ⇒ delta; epoch 0 matches a fresh store's base epoch.
+	_ = c.store.HandleFrame(map[string]string{catalog.MetaEpoch: "0"}, payload)
+	return upserts, deletes
+}