Observability: DEBUG debug-level default, greppable decision tags, log export + UDP LAN sink

Debugging the integration build on real iOS 26.5 devices over the air.

- BitLogger DEBUG default minimumLevel now .debug (was .info) so a
  tap-launched sideload surfaces .debug decision logs without the
  BITCHAT_LOG_LEVEL env var; env still wins; release unchanged (.info,
  emits nothing). Extracted defaultMinimumLevel + a test asserting it.
- Greppable bracket decision tags at .info (visible even at .info) via
  SecureLogger, ID-prefixes only, sanitized:
  [ROUTE] source-route vs flood origination (+reason) + fragment
  targeted-resync; [GW] uplink accept/reject, downlink rebroadcast/drop,
  loop skips, drain-timer; [WIFI] offer/accept/decline, AWDL connect,
  transfer start/complete+bytes, fallback+reason; [PREKEY] seal choice,
  consume, unknown-prekey open fail, bundle ingest, re-gossip;
  [SYNC] one concise summary per cycle + per-request served counts (no
  per-packet spam); [PING]/[TRACE] RTT + computed path.
- New OSLog categories: mesh, gateway, transport.
- Refactored BLESourceRouteOriginationPolicy.route -> decide returning a
  Decision enum (flood(reason)/route(hops)) so the flood reason is
  greppable and unit-tested.
- DEBUG-only in-memory ring buffer (LogExportBuffer, ~2000 lines/512KB,
  thread-safe, off main thread) + App Info "Export Logs" share sheet
  (iOS UIActivityViewController / macOS NSSavePanel), VoiceOver labels.
- DEBUG-only UDP LAN log sink (LogNetworkSink): opt-in, off by default,
  fire-and-forget, never blocks/throws; each line prefixed with the
  device nickname for demux; configured via App Info host:port fields.
  Same sanitized formatted line feeds buffer, sink, and export.

All new code is #if DEBUG; release emits nothing and ships no export/sink UI.

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

localPackages/BitLogger/Sources/LogNetworkSink.swift

@@ -0,0 +1,104 @@
+//
+// LogNetworkSink.swift
+// BitLogger
+//
+// This is free and unencumbered software released into the public domain.
+// For more information, see <https://unlicense.org>
+//
+
+#if DEBUG
+import Foundation
+#if canImport(Network)
+import Network
+#endif
+
+/// Best-effort, fire-and-forget UDP forwarder for sanitized log lines, so a
+/// sideloaded device on iOS 17+ (where `idevicesyslog` Wi-Fi streaming no
+/// longer works) can stream its logs to a LAN collector with no cable.
+///
+/// Design constraints:
+/// - DEBUG-only: never compiled into release. A privacy-first release build
+///   carries zero network-log code.
+/// - Opt-in and off by default: nothing is ever sent until a collector host
+///   is configured. An empty host means no egress.
+/// - Never blocks the caller and never throws into the app: every send runs
+///   on a private queue and failures are silently dropped. A dead or absent
+///   collector cannot affect app behavior (UDP is connectionless — datagrams
+///   to nowhere are simply lost).
+/// - Forwards the exact same sanitized text the ring buffer captures, so no
+///   secrets leave the device. Each line is prefixed with the device's label
+///   (`[nickname] …`) so one Mac-side `nc -lu <port>` can demux all devices.
+public final class LogNetworkSink {
+    public static let shared = LogNetworkSink()
+
+    /// UserDefaults keys shared with the in-app config UI (App Info sheet).
+    public static let hostDefaultsKey = "debug.logSink.host"
+    public static let portDefaultsKey = "debug.logSink.port"
+    public static let defaultPort = 9999
+    /// Where the device label is read from (the app's chosen nickname).
+    public static let nicknameDefaultsKey = "bitchat.nickname"
+
+    private let queue = DispatchQueue(label: "chat.bitchat.securelogger.netsink", qos: .utility)
+    private let defaults: UserDefaults
+    private var label = ""
+    private var enabled = false
+    #if canImport(Network)
+    private var connection: NWConnection?
+    #endif
+
+    init(defaults: UserDefaults = .standard) {
+        self.defaults = defaults
+    }
+
+    /// Re-read collector host/port and the device label from UserDefaults and
+    /// (re)build the UDP connection. Call on launch and whenever the config UI
+    /// changes. An empty host tears the sink down (no egress).
+    public func reloadConfiguration() {
+        let host = (defaults.string(forKey: Self.hostDefaultsKey) ?? "")
+            .trimmingCharacters(in: .whitespacesAndNewlines)
+        let storedPort = defaults.integer(forKey: Self.portDefaultsKey)
+        let port = storedPort > 0 ? storedPort : Self.defaultPort
+        let label = (defaults.string(forKey: Self.nicknameDefaultsKey) ?? "")
+            .trimmingCharacters(in: .whitespacesAndNewlines)
+        configure(host: host, port: port, label: label)
+    }
+
+    /// Point the sink at `host:port` with a device `label`. Empty host or an
+    /// invalid port disables egress.
+    public func configure(host: String, port: Int, label: String) {
+        queue.async {
+            self.label = label
+            #if canImport(Network)
+            self.connection?.cancel()
+            self.connection = nil
+            guard !host.isEmpty,
+                  (1...65535).contains(port),
+                  let nwPort = NWEndpoint.Port(rawValue: UInt16(port)) else {
+                self.enabled = false
+                return
+            }
+            let connection = NWConnection(host: NWEndpoint.Host(host), port: nwPort, using: .udp)
+            connection.start(queue: self.queue)
+            self.connection = connection
+            self.enabled = true
+            #else
+            self.enabled = false
+            #endif
+        }
+    }
+
+    /// Forward one already-formatted, already-sanitized line. Non-blocking;
+    /// drops silently when disabled or on any send failure.
+    func send(_ line: String) {
+        queue.async {
+            guard self.enabled else { return }
+            #if canImport(Network)
+            guard let connection = self.connection else { return }
+            let prefixed = self.label.isEmpty ? line : "[\(self.label)] \(line)"
+            guard let data = (prefixed + "\n").data(using: .utf8) else { return }
+            connection.send(content: data, completion: .idempotent)
+            #endif
+        }
+    }
+}
+#endif