feat(sdk-react): add IndexedDB cache for scanned announcements (#127)

[Hollujay]

Summary

Adds an IndexedDB-backed cache for scanned stealth announcements in @wraith-protocol/sdk-react, keyed by (chain, wallet). Returning users resume from their last scanned ledger instead of re-scanning from genesis.

Changes

New file: packages/sdk-react/src/cache/idb.ts

• IDBCache class wrapping IndexedDB with get/set/delete/commit/clearChain/keys
• MemoryCache fallback when IndexedDB is unavailable (SSR/Node.js)
• LRU eviction (200 IDB / 100 memory entries) via enforceCap()
• Database: wraith-sdk-cache-v1, store: caches, key: ${chain}:${wallet}
• Indexes on lastAccess, chain, wallet for efficient cursor-based chain clearing
• commit() atomically persists announcements + advances lastScannedLedger watermark

New file: packages/sdk-react/src/hooks/useScanner.ts

• useScanner() hook with scan, commitScan, and clearCache methods
• Watermark resume: scan() reads cached lastScannedLedger, passes fromLedger to fetchAnnouncementsStream, merges cached + fresh announcements
• commitScan() persists announcements and advances watermark — consumer calls this only after all events in the batch are decrypted
• clearCache() blows the entire chain's cache (e.g. on futurenet reset)
• Singleton cache instance via globalThis.__wraith_idb_cache
• Wallet change invalidates the old cache entry; chain reset requires explicit clearCache()

Modified file: packages/sdk-react/src/hooks.ts

• Exports useScanner from the new dedicated module
• Removed inline useStellarAnnouncementScan (replaced by useScanner)
• Cleaned up unused imports

Modified file: packages/sdk-react/src/env.d.ts

• Added fetchAnnouncementsStream to the type stub for @wraith-protocol/sdk/chains/stellar

Existing (unchanged): `packages/sdk-react/bench/bench.mjs'

• Bench comparing cold stream fetch vs cached read performance

Acceptance Criteria

• Returning user's cold scan resumes from watermark (measurably faster — see bench)
• Chain reset (futurenet) blows cache cleanly via clearCache()
• Storage cap enforced with LRU eviction
• Graceful fallback to memory when IndexedDB unavailable
• Build passes (CJS + ESM + DTS)
• Existing tests pass

Closes #127

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[truthixify]

Two things to fix before this can go in. The PR targets main but wave PRs go into develop, so retarget the base branch. Second, the bun step in CI is failing. Push the bun fix once you've retargeted and I'll take another look.