nip55: add round-trip test for AndroidSigningRateLimiterStorage encrypted-prefs adapter

[kwsantiago]

Gap

AndroidSigningRateLimiterStorage (app/src/main/kotlin/io/privkey/keep/nip55/AndroidSigningRateLimiterStorage.kt) is the SigningRateLimiterStorage adapter the Rust persistent SigningRateLimiter calls back into for every auto-sign request. It has no test coverage for its load / save / remove / clear round-trip through KeystoreEncryptedPrefs.

The ~224 lines of Kotlin velocity logic this replaced had no Android tests either (that coverage now lives in Rust), so an encoding or persistence regression in the adapter, the exact seam that gates auto-signing, would currently ship silently.

Proposed

Add an instrumented round-trip test against a real KeystoreEncryptedPrefs (or an in-memory SharedPreferences fake) covering:

• save then load returns the stored value
• load of an absent key returns null
• remove then load returns null
• clear wipes all entries
• durability: values written via save (now commit()) survive a fresh adapter instance

Mirrors #309 (the audit-chain FFI-seam round-trip test).

Context

Introduced by the persistent-limiter refactor (branch nip55-persistent-limiter-android, commit e40c899). Surfaced during PR review.