feat(iios): projection-cursor ledger + ProjectionCursorService (P9)

Adds the doc-mandated IiosProjectionCursor (ordered high-water-mark +
rolling checksum per projection/topic/partition) and a reusable
ProjectionCursorService.advance(). The checksum is a deterministic fold over
the ordered event stream, so replaying the same events reproduces it — the
KG-06 proof that replay reproduces projection outcomes.

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

packages/iios-service/prisma/schema.prisma

@@ -492,6 +492,22 @@ model IiosProcessedEvent {
   @@id([consumerName, eventId])
 }
 
+/// Projection replay cursor (P9, KG-06). Ordered high-water-mark + rolling checksum per
+/// (projection, topic, partition) proving replay reproduces the same projection outcome.
+model IiosProjectionCursor {
+  id             String   @id @default(cuid())
+  projectionName String
+  sourceTopic    String
+  partitionKey   String
+  lastEventId    String
+  lastOffset     Int      @default(0)
+  checksum       String
+  updatedAt      DateTime @updatedAt
+
+  @@unique([projectionName, sourceTopic, partitionKey])
+  @@index([partitionKey])
+}
+
 /// Idempotent-command ledger (P9). Suppresses duplicate externally-visible mutations:
 /// same (scope, command, key) replays the cached response; a different request → conflict.
 model IiosIdempotencyCommand {