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.

Package.swift

@@ -63,7 +63,11 @@ let package = Package(
                 // Only the vector fixture: declaring the whole "Noise"
                 // directory would claim its .swift test files as resources
                 // and silently drop them from compilation.
-                .process("Noise/NoiseTestVectors.json")
+                .process("Noise/NoiseTestVectors.json"),
+                // Frozen envelopes produced by the released iOS (733098bb)
+                // and Android (b7f0b33d) private-DM implementations; prove
+                // receive compatibility independently of the local generator.
+                .process("Nostr/Fixtures")
             ]
         )
     ]