refactor(run-ops): drop known-migrated read layer; residency is id-shape only

Migration/drain is deferred, so residency is decided purely by id-shape
(ownerEngine): 25-char cuid -> LEGACY, 27-char ksuid -> NEW, unclassifiable
-> LEGACY. This is behavior-preserving in production, which never injected a
custom isKnownMigrated and, with no migration, always saw the default false.

- delete knownMigratedFilter.server.ts + its test
- readThrough: drop the isKnownMigrated dep + migrated short-circuit; KEEP the
  unclassifiable->LEGACY new-then-legacy fallback
- resolveInheritedMintKind: collapse to pure ownerEngine id-shape (no deps)
- mintBatchFriendlyId: drop isKnownMigrated/isSplitEnabled from ResolveDeps
- runEngineHandlersShared: drop isKnownMigrated from EventReadDeps/readRunForEvent
  (batch-write residency probe via newReplica.batchTaskRun.findFirst is untouched)
- tests: delete injected-marker cases, keep pure id-shape assertions

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

Author: d-cs

apps/webapp/app/v3/runEngineHandlersShared.server.ts

@@ -16,9 +16,8 @@ export type EventReadDeps = {
   newReplica: PrismaReplicaClient;
   legacyReplica: PrismaReplicaClient;
   splitEnabled: boolean;
-  // Pure boundaries forwarded to read-through; production leaves them undefined
-  // so the read-through layer uses its own wired defaults. Tests inject fakes.
-  isKnownMigrated?: (runId: string) => Promise<boolean>;
+  // Pure boundary forwarded to read-through; production leaves it undefined
+  // so the read-through layer uses its own wired default. Tests inject a fake.
   isPastRetention?: (runId: string) => boolean;
 };
 
@@ -43,7 +42,6 @@ export async function readRunForEvent<S extends Prisma.TaskRunSelect>(
       newClient: deps.newReplica,
       legacyReplica: deps.legacyReplica,
       splitEnabled: deps.splitEnabled,
-      isKnownMigrated: deps.isKnownMigrated,
       isPastRetention: deps.isPastRetention,
     },
   });