Add capability bits to announce TLV

Announces now carry an optional capabilities TLV (0x05): a little-endian
bitfield with named bits for upcoming features (prekeys, wifiBulk,
gateway, groups, board, vouch, meshDiagnostics). Old clients skip the
unknown TLV; peers without it decode as nil so features can distinguish
"legacy peer" from "advertises nothing".

PeerCapabilities lives in BitFoundation with a minimal-length encoding
that preserves unknown bits for forward compatibility. Peer capabilities
are stored in the BLE peer registry on verified announce and exposed via
BLEService.peerCapabilities(_:). The local advertisement set is empty
until each feature ships its bit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

localPackages/BitFoundation/Tests/BitFoundationTests/PeerCapabilitiesTests.swift

@@ -0,0 +1,43 @@
+//
+// PeerCapabilitiesTests.swift
+// bitchatTests
+//
+// This is free and unencumbered software released into the public domain.
+// For more information, see <https://unlicense.org>
+//
+
+import Testing
+import Foundation
+@testable import BitFoundation
+
+struct PeerCapabilitiesTests {
+    @Test
+    func encodingIsMinimalAndRoundTrips() {
+        #expect(PeerCapabilities([]).encoded() == Data([0x00]))
+        #expect(PeerCapabilities.prekeys.encoded() == Data([0x01]))
+        #expect(PeerCapabilities.meshDiagnostics.encoded() == Data([0x40]))
+
+        let high = PeerCapabilities(rawValue: 1 << 9)
+        #expect(high.encoded() == Data([0x00, 0x02]))
+
+        let all: PeerCapabilities = [.prekeys, .wifiBulk, .gateway, .groups, .board, .vouch, .meshDiagnostics]
+        #expect(PeerCapabilities(encoded: all.encoded()) == all)
+        #expect(PeerCapabilities(encoded: high.encoded()) == high)
+        #expect(PeerCapabilities(encoded: PeerCapabilities([]).encoded()) == [])
+    }
+
+    @Test
+    func decodingToleratesUnknownBitsAndOversizedFields() {
+        // Unknown bits survive a round-trip untouched.
+        let unknown = PeerCapabilities(encoded: Data([0xFF, 0xFF]))
+        #expect(unknown.rawValue == 0xFFFF)
+        #expect(unknown.contains(.gateway))
+
+        // Fields longer than 8 bytes keep the low 64 bits and ignore the rest.
+        let oversized = Data([0x01] + [UInt8](repeating: 0x00, count: 7) + [0xAA, 0xBB])
+        #expect(PeerCapabilities(encoded: oversized) == .prekeys)
+
+        // Empty value decodes to no capabilities.
+        #expect(PeerCapabilities(encoded: Data()) == [])
+    }
+}