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.

bitchatTests/Nostr/Fixtures/AndroidLegacyPrivateEnvelopeB7f0b33d.json

@@ -0,0 +1 @@
+{"id":"8f4ac4680de5f972a586267bc7b6b5102ba548a34f618bdee47edd08929ee1e8","pubkey":"6981231b5745520fd982f66f485fa1b42f8f91ad25ab32242d4afb839d696b0a","created_at":1783727308,"kind":1059,"tags":[["p","c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5"]],"content":"v2:ETvtGOrkDQ2JIiiMFMdxBlrYNLh07-QKmLds0xsjkgs-v54CP4E4uoKl1L_VvKvaREg-EviPQmksd3DOYHrBVAd5E6xuRn1o9vkoss4MYV7I71mu4RfADRt77ohZaNbc-KhrmgyRTgE-WnEsqErax8LUN6GUukjkKncVA9MAmC1WUB1MI1AR8c4BQ0IOc_ubrEqi640a8AeBZaCZOYutCb3ttqNSPBxR63XErE761KNg1uiq5pgBVo8iKvLO-N2ei7IWvQhmDTapBaEU7LexeIdHDEwMdXDoAtr5Nmd54H1VN12tBvk1wXvHnENgZCOLOR5J2E1eSwrFXXBto-ohrNpBaLKIXBTPGEoepa7x0gC0Vrh1OTf4tCI7JJ5UWnkUAQFGUgPGlTRYC8MdESgEthqmdgKT3Jc0N6sylTmv6zSVx5dXqO4fvSLHC6_7it8F_V_8-uAxUYstJz65oK4F9CwOEVglUuUdfn2mN_3cMBzASLOlKvL8jbkwwo5aMBVrShGiEwDix02hfGMNMKf7OKsLlfNiBAVSPh6MQSvAWhxbDCDnWN-yHKWF4TYxbnH70X2KGl_ZD9pXTphKVIpFuRmP7UNM-01oG53NKcv9puBSxAkLbTZd122uFL_zQebxid1ukOXT8WgSB_WYJYoAlQekeu4ITryB4I60vAAzMAa4AppCUNnf6T8jwWxuC-8G0TPf18MTxpeNkzcSpPVyVE0jtLaOwsJDXs_Pg82Id03Qc-b_fWMm9V07UzGmEnMqQ1gBMLmEXHb_4Ebg5X7TdzuXy87O36CJzau7Dm5ZfoalryF-16Z4MxzQOyXb1G61yFthRsGCT6sYi-68YkhPScMf7u_BobtMuGWiMiWoBqN_IrQ_ecMHVfaeEYvpCz5NYlrE26iAktNmzCBUDNcIr6P_nHdb6I3Q1rOOmWwEF7jsLbvnU_w_82_nXE_yfdGsoly24A2wB0L0SpdnyyEWgvKWjjfS3J3vkIVW4_iM0FT0jNelANc2X_ryb3EPTmGenlqm_qRGh86PYk_R07hKYu3ULNEgLzDTijeZ9-bP23tsMXUDdJS2VR7LP5063ygVuSC0J-GL1FmQ2c-DmJaWQSeqp0NN3sCND3pSIRzwQRwChnMNjVB65mJj","sig":"c53f339b19b9de021765588b0ee4f5f1e0c2423a34d35fe2cbcc6ce89e12e2fedbdf88a0b6a9d719c2c8580002c7574f1ace93aedeab1e255b6231d4ebb6f9b2"}

bitchatTests/Nostr/Fixtures/AndroidLegacyPrivateEnvelopeB7f0b33dGenerator.patch.txt

@@ -0,0 +1,29 @@
+diff --git a/app/src/test/kotlin/com/bitchat/android/nostr/NostrProtocolTest.kt b/app/src/test/kotlin/com/bitchat/android/nostr/NostrProtocolTest.kt
+index a5bd956..544a29b 100644
+--- a/app/src/test/kotlin/com/bitchat/android/nostr/NostrProtocolTest.kt
++++ b/app/src/test/kotlin/com/bitchat/android/nostr/NostrProtocolTest.kt
+@@ -10,0 +11,21 @@ class NostrProtocolTest {
++    @Test
++    fun emitCrossPlatformFixtures() {
++        val sender = NostrIdentity.fromPrivateKey(
++            "0000000000000000000000000000000000000000000000000000000000000001"
++        )
++        val recipient = NostrIdentity.fromPrivateKey(
++            "0000000000000000000000000000000000000000000000000000000000000002"
++        )
++        val giftWrap = NostrProtocol.createPrivateMessage(
++            content = "legacy fixture from Android b7f0b33d",
++            recipientPubkey = recipient.publicKeyHex,
++            senderIdentity = sender
++        ).single()
++
++        assertEquals(NostrKind.GIFT_WRAP, giftWrap.kind)
++        assertEquals(listOf(listOf("p", recipient.publicKeyHex)), giftWrap.tags)
++        println("BITCHAT_FIXTURE_EVENT=${gson.toJson(giftWrap)}")
++        println("BITCHAT_FIXTURE_RECIPIENT_PRIVATE_KEY=${recipient.privateKeyHex}")
++        println("BITCHAT_FIXTURE_SENDER_PUBLIC_KEY=${sender.publicKeyHex}")
++    }
++
+     @Test
+     fun decryptPrivateMessage_acceptsAuthenticatedSeal() {
+         val sender = NostrIdentity.generate()

bitchatTests/Nostr/Fixtures/AndroidLegacyPrivateEnvelopeB7f0b33dMetadata.json

@@ -0,0 +1,10 @@
+{
+  "android_commit": "b7f0b33d3a267c770d3d5a65ee2d8c7e755450db",
+  "generator": "NostrProtocol.createPrivateMessage",
+  "generator_patch": "AndroidLegacyPrivateEnvelopeB7f0b33dGenerator.patch.txt",
+  "generator_patch_sha256": "e7ad29d6a247638cc16fb2ec06937266e6a03e7a115ae00913330a86a88fa56a",
+  "gradle_test": "ANDROID_HOME=/opt/homebrew/share/android-commandlinetools ANDROID_SDK_ROOT=/opt/homebrew/share/android-commandlinetools JAVA_HOME=/opt/homebrew/opt/openjdk@17/libexec/openjdk.jdk/Contents/Home ./gradlew testDebugUnitTest --tests com.bitchat.android.nostr.NostrProtocolTest.emitCrossPlatformFixtures --info",
+  "fixture_sha256": "d2df3d0b7ffd84c5cb25e0b3f4a89ad14f72a105bddadab77081f98c661b080e",
+  "recipient_private_key": "0000000000000000000000000000000000000000000000000000000000000002",
+  "sender_public_key": "79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798"
+}

bitchatTests/Nostr/Fixtures/LegacyPrivateEnvelope733098bb.json

@@ -0,0 +1 @@
+{"content":"v2:SorfnTaoQ_Rv0XLKA8b3FZojgaFvnWgx66JR6Gj20ztnorQyL-hUYBZwAa5ohFC6ioR9hlJARJm0cgNTvWgSZuNTcfSAvoMdSG6mXK73kzsp99x351zUB_lc1_5ZXm0qqePAEz3Dl5jj5EsfAb8ZzsCd-BZENCsTwqjqC_hCFl7RitlRfIL2Tq_n4TUFEFandDCplNz3W4L7V07V89aGEoUlhzcwPU44BcxfvQjeqVKq0IRf2AetypoOduPrjSqkv674ubWaRcHtw3Asrqgo5XSYbpOlSk1PF_TttrQSPZGViNe5MuiK2P-7d-XqKXuDf_bUgAzW884KXogbct-wtIJZJbVM-utMd-dHrpC1mY81lgpS4_kPuhj0Z6Ro9hU5nCcEk2K4_vNoSM9m9QbcXP34h47qSPsw9ikmz8UoD00-1fXQVB4YJcBVUSVI06IzbZEWulo8SPXvQ4pJjV3nwPgYqRgwnrNWMNfeuKojlF8yA17UNOWvD2U7r1cs84HL8dxztzX9NdN0DGxvjMILvt3D4eWrMbcSrsIkBgyvV-uskEPd4eX3fc9GmX8MPkMgxRErcxbuq6JBUNXikOJXNH_qspOt4UIw5dCIajrHsKycKd8A_3rSgLEQteirOWMGaD2gOJEzpbe4iT72dmPkvRq7k-wDjLbO5dOSuUvpFM-ipkB07ATJz_1uUcRpl8fD_oSlcdAzdPjGKG5Y-tNv3AcUstkqCi52E5x-aO8EDUNBFi_OCbD86nkTUv1jOpAQwejowSiiOnCZ91zUEg5pRQyhBV0Ozib-j0Wf8S8VAz9M8bqQQaKedPCEowDn6csOKbFdBtqSeROf1XWKCkm1mSJGHWW9V44MiMiHebVrRUpbP0PqvxiIeJE0","created_at":1783710443,"id":"46425f8d4e8007af43abb67b3668b59604bd34c86dee1b1c3702559086927cb9","kind":1059,"pubkey":"960e391e314a7fb00bbdd85eccb0a93c17e981b6fed38487cf891f1ed6b66aeb","sig":"975c88c1c4d11f0b623603f8ecc7c181fea7385e8feacdb4a64a6b4f7536b1337ff556aee165ca337c9ec501cfab3e9703026c6df2b12bb8c702378875f00722","tags":[["p","1c108500bf53da288d30530718bac4bf80d661d1fe38854060c5b5c79eb77755"]]}

bitchatTests/Nostr/Fixtures/LegacyPrivateEnvelope733098bbRecipientKey.json

@@ -0,0 +1 @@
+{"recipient_private_key":"8355a5c110cdfef2e644f4ad5d51c39f253b2c2c80ebb6856379fb16531dc1fa"}