feat(companion): backup envelope + NIP-46 signer crypto in the native core

Extends archy-fips-core with the two companion-release features' crypto
(#128, #139), same JNI-over-JSON contract as the mesh surface:

backup.rs — the ADR-005 encrypted-backup envelope, byte-compatible with
the node's backup code (Argon2id default params + ChaCha20-Poly1305,
blob = base64(salt||nonce||ct)); decrypt ignores extra envelope fields
so node backups read here too. Round-trip, tamper, wrong-passphrase and
cross-shape tests included.

nostr.rs — the phone-side remote-signer crypto: nsec/npub bech32 keys,
BIP340 schnorr event signing (NIP-01 id serialization), NIP-44 v2
payloads (HKDF-SHA256 + ChaCha20 + HMAC-SHA256, both padding prefixes),
NIP-04 fallback, nostrconnect:// parsing with repeated relay params.
Verified against the official NIP-44 vectors (conversation keys, message
keys, padded lengths, byte-exact encrypt vectors), the BIP-340 reference
sign vectors, and round-trip/tamper/failure tests. secp256k1 0.29 note:
Keypair::public_key() is the 33-byte compressed key — x-only pubkeys
must go through .x_only_public_key().0 (one real bug the vectors caught).

JNI glue adds com.archipelago.app.NativeCore: backupEncrypt/Decrypt,
nostrGenerateSecret/SecretFromAny/ParseConnectUri/SignEvent and the
NIP-44/NIP-04 cipher pairs. Android arm64 build verified via cargo-ndk
(7.2 MB .so, +0.4 MB for both modules). Host: cargo test 23/23, clippy clean.

Android/rust/archy-fips-core/src/lib.rs

@@ -11,7 +11,9 @@
 //! JSON-over-strings, mirroring the myco / nostr-vpn embedding pattern:
 //! `generateIdentity`, `deriveIdentity`, `start`, `stop`, `isRunning`.
 
+pub mod backup;
 pub mod mesh;
+pub mod nostr;
 
 #[cfg(target_os = "android")]
 mod jni_glue;