Relabel private Nostr envelopes honestly in docs; harden legacy envelope validation (#1480)

Split-out safe half of #1437 (the kind-1402 wire migration stays held for Android coordination). Docs (README/WHITEPAPER/PRIVACY_POLICY/privacy-assessment) now describe the actual proprietary DM construction — kind 1059 gift wrap carrying XChaCha20-Poly1305 with a 24-byte nonce, base64url v2: framing, and an HKDF that borrows the nip44-v2 info label but is not the NIP-44 key schedule — instead of claiming NIP-17/44/59.

Hardens the existing legacy inbound path: 64 KiB ciphertext cap before decode (~46x the largest producible legacy envelope), outer kind/recipient-tag/signature binding, tagless kind-13 seal binding, unsigned kind-14 inner binding, inner tags restricted to the two shapes deployed clients emit (verified against every historical iOS release and a fixture frozen from Android production), SecRandomCopyBytes failure now throws, non-UTF-8 plaintext now throws instead of returning empty. Adds frozen cross-platform fixtures with hash-pinned generators; interop-reviewed with no rejection surface for deployed clients.

README.md

@@ -19,7 +19,7 @@ This project is released into the public domain. See the [LICENSE](LICENSE) file
 - **Intelligent Message Routing**: Automatically chooses best transport (Bluetooth → Nostr fallback)
 - **Decentralized Mesh Network**: Automatic peer discovery and multi-hop message relay over Bluetooth LE
 - **Privacy First**: No accounts, no phone numbers, no persistent identifiers
-- **Private Message End-to-End Encryption**: [Noise Protocol](https://noiseprotocol.org) for mesh, NIP-17 for Nostr
+- **Private Message End-to-End Encryption**: [Noise Protocol](https://noiseprotocol.org) for mesh, BitChat private envelopes for Nostr fallback
 - **IRC-Style Commands**: Familiar `/slap`, `/msg`, `/who` style interface
 - **Universal App**: Native support for iOS and macOS
 - **Emergency Wipe**: Triple-tap to instantly clear all data
@@ -44,9 +44,15 @@ BitChat uses a **hybrid messaging architecture** with two complementary transpor
 - **Global Reach**: Connect with users worldwide via internet relays
 - **Location Channels**: Geographic chat rooms using geohash coordinates
 - **290+ Relay Network**: Distributed across the globe for reliability
-- **NIP-17 Encryption**: Gift-wrapped private messages for internet privacy
+- **BitChat Private Envelopes**: App-specific encrypted private messages over Nostr relays
 - **Ephemeral Keys**: Fresh cryptographic identity per geohash area
 
+BitChat's private-envelope format is proprietary and is **not** NIP-17,
+NIP-44, or NIP-59 compatible. It uses Nostr as a relay transport but only
+interoperates with BitChat clients: private payloads travel inside kind-1059
+events whose `v2:`-prefixed content is a BitChat-specific XChaCha20-Poly1305
+construction, not NIP-44 encryption.
+
 ### Channel Types
 
 #### `mesh #bluetooth`
@@ -80,7 +86,7 @@ Private messages use **intelligent transport selection**:
 2. **Nostr Fallback** (when Bluetooth unavailable)
 
    - Uses recipient's Nostr public key
-   - NIP-17 gift-wrapping for privacy
+   - BitChat's app-specific private-envelope encryption
    - Routes through global relay network
 
 3. **Smart Queuing** (when neither available)