Encrypt private media before BLE fragmentation (#1434)

Closes the last cleartext private-content path over BLE: private DM images/voice were sent as plaintext signed fileTransfer packets, TTL-relayed across the mesh, so every relay saw the full bytes. Now the complete BitchatFilePacket is encrypted as a single Noise AEAD message (inner type 0x20, matching Android) and the opaque ciphertext is fragmented. Adds an authenticated in-session capability proof (0x21 TLV: capabilities + Ed25519 key), TOFU-style downgrade pinning, a per-send consent dialog for the signed-cleartext fallback to legacy peers, and a cancellation/admission registry so cancel/delete cannot race a deferred cleartext send.

Android wire constants (0x20 / 0x21 / capability bit 8) confirmed shipping. The 256-fragment preflight cap applies only to the directed fileTransfer migration fallback; encrypted media to capable peers uses the full receiver ceiling.

Rebased over #1428/#1349: identity reads go through BLELocalIdentityStateStore; the session-bound authenticated signing-key check and the announce-path TOFU pin are kept as complementary checks. Full local suite green (1744+197 tests).

bitchatTests/Services/BLENoisePayloadFactoryTests.swift

@@ -1,5 +1,6 @@
 import Foundation
 import Testing
+import BitFoundation
 @testable import bitchat
 
 struct BLENoisePayloadFactoryTests {
@@ -31,4 +32,63 @@ struct BLENoisePayloadFactoryTests {
 
         #expect(payload == Data([NoisePayloadType.verifyChallenge.rawValue, 0xCA, 0xFE]))
     }
+
+    @Test
+    func privateFilePayloadPrefixesCanonicalFilePacket() throws {
+        let content = Data("%PDF-secret".utf8)
+        let file = BitchatFilePacket(
+            fileName: "secret.pdf",
+            fileSize: UInt64(content.count),
+            mimeType: "application/pdf",
+            content: content
+        )
+
+        let payload = try #require(BLENoisePayloadFactory.privateFile(file))
+
+        #expect(payload.first == 0x20, "Encrypted files must use Android's deployed wire value")
+        let decoded = try #require(BitchatFilePacket.decode(Data(payload.dropFirst())))
+        #expect(decoded.fileName == "secret.pdf")
+        #expect(decoded.mimeType == "application/pdf")
+        #expect(decoded.content == content)
+    }
+
+    @Test
+    func androidB7f0b33PrivateFilePlaintextFixtureIsByteCompatible() throws {
+        // Runtime-emitted by Android commit b7f0b33d from
+        // BitchatFilePacket("a.txt", 3, "text/plain", [01, 02, 03]) and
+        // NoisePayload(type = FILE_TRANSFER, data = file.encode()).encode().
+        let fixtureHex = "20010005612e7478740200040000000303000a746578742f706c61696e0400000003010203"
+        let fixture = try #require(Data(hexString: fixtureHex))
+
+        let typed = try #require(NoisePayload.decode(fixture))
+        #expect(typed.type == .privateFile)
+        let file = try #require(BitchatFilePacket.decode(typed.data))
+        #expect(file.fileName == "a.txt")
+        #expect(file.fileSize == 3)
+        #expect(file.mimeType == "text/plain")
+        #expect(file.content == Data([0x01, 0x02, 0x03]))
+        #expect(BLENoisePayloadFactory.privateFile(file) == fixture)
+    }
+
+    @Test
+    func prereleasePrivateFileTypeCanonicalizesOnDecode() throws {
+        let encoded = Data([NoisePayloadType.prereleasePrivateFileRawValue, 0xCA, 0xFE])
+        let decoded = try #require(NoisePayload.decode(encoded))
+
+        #expect(decoded.type == .privateFile)
+        #expect(decoded.data == Data([0xCA, 0xFE]))
+        #expect(decoded.encode().first == 0x20)
+    }
+
+    @Test
+    func authenticatedPeerStateUsesPermanent0x21Type() throws {
+        let state = AuthenticatedPeerStatePacket(
+            capabilities: .privateMedia,
+            signingPublicKey: Data(repeating: 0x77, count: 32)
+        )
+        let encoded = try #require(BLENoisePayloadFactory.authenticatedPeerState(state))
+
+        #expect(encoded.first == 0x21)
+        #expect(AuthenticatedPeerStatePacket.decode(from: Data(encoded.dropFirst())) == state)
+    }
 }