fix: iOS 26 UDID fallback — use cached pairing file UDID when C library fails

[chenbusi123]

Problem

On iOS 26 (26.3+), fetch_first_device() consistently fails because the bundled C library (rusty_libimobiledevice, last updated Sept 2023) cannot communicate with iOS 26 lockdownd over the VPN tunnel. This causes SideStore to report:

"could not determine this device UDID. Please replace your pairing using iloader."

This blocks Apple ID sign-in, app refresh, and all sideloading operations.

Related SideStore issues: #1305, #1312, #1316, #1320, #1322, #1324

Root Cause

The rusty_libimobiledevice C bindings (from 2023) implement an outdated lockdown protocol. Upstream libimobiledevice has since added critical changes:

• SHA256 instead of SHA1 for pairing certificate signatures
• ProductVersion/DeviceClass queries moved into lockdownd_client_new
• Only query ProductVersion if talking to actual lockdownd

Fix

This PR adds a pairing-file-based UDID cache as fallback:

1. muxer.rs: Cache UDID from pairing file after parsing
2. device.rs: fetch_first_device() tries C library first, falls back to cached UDID

Minimal safe change: only activates when C library fails. Uses existing once_cell dependency.

Related

Companion branch for tracking upstream libimobiledevice changes: https://github.com/chenbusi123/rusty_libimobiledevice/tree/ios26-fix