feat: add streamed subgraph user announcement scan

[marcomariscal]

Summary

• add a subgraph-first async generator for scanning user announcements page by page
• yield matches incrementally with scannedCount, nextCursor, and snapshotBlock
• document the happy-path usage and add a simple React streaming example

Changes

• reuse the bounded page API from getAnnouncementsPageUsingSubgraph
• extract shared user-filtering helpers so eager and streaming paths stay aligned
• enforce strict chain-recency ordering for streamed batches and cover resume/empty-batch behavior with tests

Test Plan

• ~/.bun/bin/bunx tsc --noEmit
• ~/.bun/bin/bun test src/lib/actions/getAnnouncementsForUser/getAnnouncementsForUser.test.ts src/lib/actions/scanAnnouncementsForUserUsingSubgraph/scanAnnouncementsForUserUsingSubgraph.test.ts src/lib/actions/getAnnouncementsUsingSubgraph/subgraphHelpers.test.ts
• ~/.bun/bin/bunx biome check README.md src/lib/actions/getAnnouncementsForUser/getAnnouncementsForUser.ts src/lib/actions/getAnnouncementsForUser/getAnnouncementsForUser.test.ts src/lib/actions/getAnnouncementsForUser/types.ts src/lib/actions/scanAnnouncementsForUserUsingSubgraph/types.ts src/lib/actions/scanAnnouncementsForUserUsingSubgraph/scanAnnouncementsForUserUsingSubgraph.ts src/lib/actions/scanAnnouncementsForUserUsingSubgraph/scanAnnouncementsForUserUsingSubgraph.test.ts src/lib/actions/index.ts src/lib/stealthClient/createStealthClient.ts src/lib/stealthClient/types.ts examples/scanAnnouncementsForUserUsingSubgraph/index.tsx examples/scanAnnouncementsForUserUsingSubgraph/package.json examples/scanAnnouncementsForUserUsingSubgraph/tsconfig.json examples/scanAnnouncementsForUserUsingSubgraph/vite.config.ts examples/scanAnnouncementsForUserUsingSubgraph/index.html examples/scanAnnouncementsForUserUsingSubgraph/README.md

Part of #91