feat(stellar): add announcement cache with MemoryCache and IndexedDBC…

[AlonsoFi]

Closes #21

Implements the full cache layer from issue #12:

Cache abstraction: AnnouncementCache interface with get, put, getLastSeen, setLastSeen, clear
MemoryCache: LRU eviction by ledger bucket, deduplication by stealthAddress, network isolation
IndexedDBCache: batched writes (single IDBTransaction per put), 50 MB LRU eviction, schema versioning via
onupgradeneeded
fetchAnnouncements: delta-fetch wired — resumes from lastSeen ledger+cursor, merges with cache, supports bypassCache
option
Tests (cache.test.ts): eviction, network isolation, idempotency, lastSeen — MemoryCache
Tests (cache.idb.test.ts): full IndexedDBCache suite via fake-indexeddb + version migration test
Benchmarks (cache.bench.ts): put/get on 1000 announcements, incremental delta — run with pnpm vitest bench

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[truthixify]

Cache layer additions (MemoryCache + IndexedDB) are what we want for issue #21. Same rebase blocker as several other PRs this round, your branch is pre-develop. Rebase, keep develop's versions for files you didn't intend to touch, keep your new cache.ts / cache.test.ts / cache.idb.test.ts / cache.bench.ts additions.

git fetch origin
git rebase origin/develop
git push --force-with-lease

Thanks @AlonsoFi.