fix(apple): define in6_ifreq locally so iOS targets build with libc 0.2.190 - #192

Phase 0 of docs/relay-plan.md needs a phone column: the phone itself on
mobile data, behind the carrier's NAT alone, not a laptop on its hotspot.

- tools/relay-probe: the core moves into a library (src/lib.rs): tickets,
  the post-quantum endpoint with no port mapping, the dialler's ping loop,
  path names and the summary table. The command keeps its flags and its
  output, --bind included; `listen` stays in main.rs.
- apps/phone: a Network test under More and on the Connect screen, which
  dials with that library on a tokio runtime of its own. Paste the ticket
  from the Mac, Start/Stop, the path as it changes, the command's table and
  log. The phone's probe key is a file in the app's container until the
  Keychain lands; its id is shown and copied for the relay's allow list.
  The screen stays awake during a run.
- libc held at 0.2.189 for iOS: netdev 0.46 (iroh's netwatch) uses
  libc::in6_ifreq, which 0.2.190 made macOS-only (shellrow/netdev#192).

Checked in the iOS simulator against `relay-probe listen --relay
https://relay2.kvad.eu` on this Mac: opened on the relay, LAN after
0.12 s, 0.9 ms median. Builds for aarch64-apple-ios too.

* Run the relay probe from the iPhone: a Network test screen

Phase 0 of docs/relay-plan.md needs a phone column: the phone itself on
mobile data, behind the carrier's NAT alone, not a laptop on its hotspot.

- tools/relay-probe: the core moves into a library (src/lib.rs): tickets,
  the post-quantum endpoint with no port mapping, the dialler's ping loop,
  path names and the summary table. The command keeps its flags and its
  output, --bind included; `listen` stays in main.rs.
- apps/phone: a Network test under More and on the Connect screen, which
  dials with that library on a tokio runtime of its own. Paste the ticket
  from the Mac, Start/Stop, the path as it changes, the command's table and
  log. The phone's probe key is a file in the app's container until the
  Keychain lands; its id is shown and copied for the relay's allow list.
  The screen stays awake during a run.
- libc held at 0.2.189 for iOS: netdev 0.46 (iroh's netwatch) uses
  libc::in6_ifreq, which 0.2.190 made macOS-only (shellrow/netdev#192).

Checked in the iOS simulator against `relay-probe listen --relay
https://relay2.kvad.eu` on this Mac: opened on the relay, LAN after
0.12 s, 0.9 ms median. Builds for aarch64-apple-ios too.

* Link the iPhone app for iOS 17 when cargo builds it alone

A bare cargo build for aarch64-apple-ios linked for iOS 10, older than
Skia's prebuilt objects (iOS 12), and failed on ___chkstk_darwin; Xcode
builds were unaffected, since it passes project.yml's deployment target.
apps/phone/.cargo/config.toml sets the same 17.0 for cargo, without
overriding Xcode's.